Add support for keyfiles with -ethKeystorePath, update flag descriptions, flagset output to stdout

[eliteprox]

What does this pull request do? Explain your changes. (required)

• Updates various flag descriptions that a file can be used in place of a string.
• Changes the default flagset output to os.StdOut. This has the effect of enabling users to grep --help output,
• Add support for keystore file with -ethKeystorePath. This can be used alternatively to ethAcctAddr
  • ETH address is parsed from keyfile. -ethAcctAddr is optional, but if provided the address must match with the one found in the keyfile.

Specific updates (required)

• Updated flag descriptions for OrchSecret, EthAcctAddr, EthPassword, EthKeystorePath and PricePerBroadcaster
• Changed command line output for flagset to os.StdOut
• Parse ETH address from keyfile when a file is provided to -ethKeystorePath
• Validates ethAcctAddr and keystore address match or that ethAcctAddr is not provided, before using address from keystore
• Added test coverage

How did you test each of these updates (required)
I tested the ethKeystorePath and ethAcctAddr flags in the following ways:

• With a directory and with a valid file. Eth account is properly found from keystore
• When set to a directory: keystoreDir was set to same path and ethAcctAddr flag still works for selecting eth account.
• When set to a valid key file: keystoreDir was set to parent directory of the file and ETH address is parsed. If -ethAcctAddr flag is also provided, the address parsed from keyfile must match the ethAcctAddr, otherwise error: -ethKeystorePath and -ethAcctAddr were both provided, but ethAcctAddr does not match the address found in keystore
• With a missing file or directory: provided -ethKeystorePath was not found
• With invalid json in file: error opening keystore

Does this pull request close any open issues?
fix #2689 cli help is printed to stderr instead of stdout
fix #2690 cli help didn't mention -ethPassword and -orchSecret can be file path
fix #2691 Usage for -ethKeystorePath option is confusing

Checklist:

• Read the contribution guide
• make runs successfully
• All tests in ./test.sh pass
• README and other documentation updated
• Pending changelog updated

[codecov]

Codecov Report

Merging #2713 (e71c828) into master (f07d69d) will decrease coverage by 0.02593%.
The diff coverage is 42.30769%.

❗ Current head e71c828 differs from pull request most recent head 0710709. Consider uploading reports for the commit 0710709 to get more accurate results

@@                 Coverage Diff                 @@
##              master       #2713         +/-   ##
===================================================
- Coverage   56.34825%   56.32232%   -0.02593%     
===================================================
  Files             88          88                 
  Lines          19147       19186         +39     
===================================================
+ Hits           10789       10806         +17     
- Misses          7767        7788         +21     
- Partials         591         592          +1     

Impacted Files	Coverage Δ
common/util.go	60.29851% <0.00000%> (-1.28686%)	⬇️
cmd/livepeer/starter/starter.go	4.29338% <43.58974%> (+1.44149%)	⬆️
cmd/livepeer/livepeer.go	47.34043% <83.33333%> (-0.25315%)	⬇️

---

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Impacted Files	Coverage Δ
common/util.go	60.29851% <0.00000%> (-1.28686%)	⬇️
cmd/livepeer/starter/starter.go	4.29338% <43.58974%> (+1.44149%)	⬆️
cmd/livepeer/livepeer.go	47.34043% <83.33333%> (-0.25315%)	⬇️

[joegraviton]

Thanks for fixing my 3 issues in batch 👍

[leszko]

@eliteprox Thanks for the PR. Mark as ready when you think you finalized it. Then, we'll review and comment on this.

[eliteprox]

@leszko This PR is ready for review.

[leszko]

@leszko This PR is ready for review.

Perfect. Thank you. I'll review it.

[leszko]

I added a few comments. Most of them are minor.

From the more important ones, I think we should fail-fast in the following scenarios:

• EthKeystorePath points to a non-existing file
• EthAcctAddr and EthKeystorePath are both defined but they point different ETH addresses

[leszko]

Added some super minor comments. Please take a look and address and think it's ready for the merge. Good work 👍

[leszko]

@eliteprox when you address all comments, please click "Re-request review"

[leszko]

Added one more comment, could you check?

[leszko]

LGTM, thanks for all the changes @eliteprox

I'll merge as soon as the CI build is completed.