Orchestrator: add flag to skip the availability check on startup

[stronk-dev]

What does this pull request do? Explain your changes. (required)
When booting an Orchestrator, an availability check is performed by sending a Ping GRPC request to the configured service address
On certain setups local loopback might not be available, causing the test to fail even though the Orchestrator is available from remote connections

This change adds a testOrchAvail flag to explicitly disable the startup check

How did you test each of these updates (required)
Going to use the binaries built by this PR to test this on my Singapore node, which exhibits this issue

[codecov]

Codecov Report

Merging #2928 (588bdbd) into master (37447ea) will decrease coverage by 0.01354%.
The diff coverage is 17.64706%.

❗ Current head 588bdbd differs from pull request most recent head bde3974. Consider uploading reports for the commit bde3974 to get more accurate results

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #2928         +/-   ##
===================================================
- Coverage   56.35345%   56.33991%   -0.01354%     
===================================================
  Files             89          89                 
  Lines          19454       19464         +10     
===================================================
+ Hits           10963       10966          +3     
- Misses          7882        7889          +7     
  Partials         609         609                 

Files	Coverage Δ
cmd/livepeer/livepeer.go	50.74627% <100.00000%> (+0.74627%)	⬆️
cmd/livepeer/starter/starter.go	4.70685% <0.00000%> (-0.02737%)	⬇️

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Files	Coverage Δ
cmd/livepeer/livepeer.go	50.74627% <100.00000%> (+0.74627%)	⬆️
cmd/livepeer/starter/starter.go	4.70685% <0.00000%> (-0.02737%)	⬇️

[stronk-dev]

Currently testing on Singapore. Flag seems to have worked, let's see if work still comes in...

[thomshutt]

Awesome, thanks @stronk-dev! Let me know once you're happy with the testing and we can merge

[stronk-dev]

Seems fine so far, any issues with the naming or comments? Wasn't sure what to group the new flag as

[thomshutt]

Naming looks good, the only bit I'd maybe change would be the flag itself to be the full version - startupAvailabilityCheck or testOrchAvailability? Internal variables etc. can stay as they are though

[leszko]

Thanks for the PR @stronk-dev Looks good to me.

Agree with @thomshutt that we could rename the flag. Other than that LGTM. @thomshutt Feel free to merge this PR.

[thomshutt]

LGTM @stronk-dev, could you update the pending changelog please and then I'll merge