Allow default price with cli option 20

[eliteprox]

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

This change allows us to set the base price per pixel with option 20. Set price for broadcaster in livepeer_cli. It is simpler to use than 13. Set orchestrator config.

Specific updates (required)

• Updates input validation for livepeer_cli to allow default to be used in place of eth address
• Defaults no input for eth address to default

How did you test each of these updates (required)

• eth address field empty, set price and verified the base price per pixel was set
• invalid eth address and confirmed Failed to validate input
• valid eth address and confirmed it was accepted.
• Ran applicable tests

Does this pull request close any open issues?
None

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 #2911 (fd7f7ec) into master (68c80a1) will not change coverage.
The diff coverage is 0.00000%.

Additional details and impacted files

@@              Coverage Diff              @@
##              master       #2911   +/-   ##
=============================================
  Coverage   56.54162%   56.54162%           
=============================================
  Files             89          89           
  Lines          19460       19460           
=============================================
  Hits           11003       11003           
  Misses          7849        7849           
  Partials         608         608           

Files	Coverage Δ
cmd/livepeer_cli/wizard_transcoder.go	0.00000% <0.00000%> (ø)

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Files	Coverage Δ
cmd/livepeer_cli/wizard_transcoder.go	0.00000% <0.00000%> (ø)

[thomshutt]

LGTM, thanks for the change!