Add startup flag to specify ai runner docker image

[eliteprox]

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

Adds the optional -aiRunnerImage flag which can be used to specify the version of ai runner docker image.

Usage: -aiRunner livepeer/ai-runner:0.0.1

If the flag is not set, then livepeer/ai-runner:latest is used by default.

This will help node operators with versioning to prevent accidental upgrades of ai-runner.

Specific updates (required)

• Adds startup flag to starter.go
• Adds config flag to livepeer.go

How did you test each of these updates (required)

• Started go-livepeer as aiWorker with -aiRunner livepeer/ai-runner:0.0.1. Verified this version was launched via docker ps
• Started go-livepeer without -aiRunner flag and verified livepeer/ai-runner:latest was launched via docker ps.

Does this pull request close any open issues?

LIV-382

Checklist:

• [x] Read the contribution guide
• [x] make runs successfully
• [x] All tests in ./test.sh pass
• [ ] README and other documentation updated
• [x] Pending changelog updated