Introduce configurable blacklist

[mjh1]

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

Implement a configurable orchestrator blacklist. This has been requested so that we can exclude permanently poor performing orchestrators.

Specific updates (required)

The blacklist is set via a command line argument (comma separated list of addresses) and is eventually used by the discovery logic when choosing an orchestrator.

How did you test each of these updates (required)

• existing unit tests adapted to cover this new scenario.
• testing with a locally running version of B/O/T to confirm the changes work as expected in the overall app. Tested blacklisting my single O and observed no segments then being sent.

Does this pull request close any open issues?

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 #2827 (1bf6edb) into master (2b487db) will decrease coverage by 0.00267%.
The diff coverage is 62.50000%.

@@                 Coverage Diff                 @@
##              master       #2827         +/-   ##
===================================================
- Coverage   56.74404%   56.74137%   -0.00267%     
===================================================
  Files             88          88                 
  Lines          19128       19143         +15     
===================================================
+ Hits           10854       10862          +8     
- Misses          7679        7686          +7     
  Partials         595         595                 

Impacted Files	Coverage Δ
cmd/livepeer/starter/starter.go	4.26288% <0.00000%> (-0.02666%)	⬇️
discovery/db_discovery.go	70.32967% <100.00000%> (+0.10908%)	⬆️
discovery/discovery.go	92.12598% <100.00000%> (+0.45931%)	⬆️

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Impacted Files	Coverage Δ
cmd/livepeer/starter/starter.go	4.26288% <0.00000%> (-0.02666%)	⬇️
discovery/db_discovery.go	70.32967% <100.00000%> (+0.10908%)	⬆️
discovery/discovery.go	92.12598% <100.00000%> (+0.45931%)	⬆️