cmd: Use -gateway consistently

[j0sh]

Avoid references to deprecated CLI flags. See #3053

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

Changes most -broadcaster references (as found by rg "\-broadcaster") to -gateway

Specific updates (required)

• No functional changes
• A bit of import rearranging done automatically by go-fmt

How did you test each of these updates (required)

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

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 57.40917%. Comparing base (a5cdcc6) to head (882365e).

Additional details and impacted files

@@              Coverage Diff              @@
##              master       #3104   +/-   ##
=============================================
  Coverage   57.40917%   57.40917%           
=============================================
  Files             92          92           
  Lines          15771       15771           
=============================================
  Hits            9054        9054           
  Misses          6113        6113           
  Partials         604         604           

Files	Coverage Δ
cmd/livepeer/starter/starter.go	7.90514% <0.00000%> (ø)

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Files	Coverage Δ
cmd/livepeer/starter/starter.go	7.90514% <0.00000%> (ø)