cmd: Make CLI flag casing consistent for dataDir flag

[thomshutt]

What does this pull request do? Explain your changes. (required)
Adds dataDir as a CLI option and marks datadir as deprecated, making all our CLI options now consistently camel-cased.

How did you test each of these updates (required)

• Added unit test
• Ran binary with --help and checked it printed both options
• Ran binary using both options and checked data dir specific was created

Does this pull request close any open issues?

• command-line flags should be case-consistent #1362

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 #2437 (83960be) into master (06b3cdc) will decrease coverage by 0.06381%.
The diff coverage is 100.00000%.

@@                 Coverage Diff                 @@
##              master       #2437         +/-   ##
===================================================
- Coverage   55.10277%   55.03896%   -0.06381%     
===================================================
  Files             93          94          +1     
  Lines          19460       19637        +177     
===================================================
+ Hits           10723       10808         +85     
- Misses          8140        8232         +92     
  Partials         597         597                 

Impacted Files	Coverage Δ
cmd/livepeer/livepeer.go	46.32768% <100.00000%> (ø)
discovery/db_discovery.go	68.05556% <0.00000%> (+1.04166%)	⬆️

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[AlexKordic]

Looks good