Fix backwards compatibility of livepeer_cli with prior livepeer version

[eliteprox]

What does this pull request do? Explain your changes. (required)
Resolves a bug when opening livepeer_cli version 0.5.35 with older versions of livepeer that did not have the price per broadcaster functionality.

Specific updates (required)

• Checks http response from /status for missing field BroadcasterPrices before reading it to prevent runtime error on startup.
• Returns a blank string for the field if not provided by livepeer /status

How did you test each of these updates (required)

• Tested livepeer_cli with old version of livepeer to confirm runtime issue is resolved.
• Tested livepeer_cli with current version and with broadcaster pricing set to confirm it still works as designed.
• No changes to option 20, but did confirm there's no backwards compatibility issues with that.

Does this pull request close any open issues?
Fixes #2697

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 #2698 (fe09753) into master (b7975e5) will increase coverage by 0.00979%.
The diff coverage is 0.00000%.

@@                 Coverage Diff                 @@
##              master       #2698         +/-   ##
===================================================
+ Coverage   56.33626%   56.34605%   +0.00979%     
===================================================
  Files             88          88                 
  Lines          19128       19130          +2     
===================================================
+ Hits           10776       10779          +3     
+ Misses          7764        7763          -1     
  Partials         588         588                 

Impacted Files	Coverage Δ
cmd/livepeer_cli/wizard_stats.go	0.00000% <0.00000%> (ø)
discovery/db_discovery.go	70.75812% <0.00000%> (+1.08303%)	⬆️

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Impacted Files	Coverage Δ
cmd/livepeer_cli/wizard_stats.go	0.00000% <0.00000%> (ø)
discovery/db_discovery.go	70.75812% <0.00000%> (+1.08303%)	⬆️

[leszko]

@eliteprox Thanks a lot for the PR. LGTM