Remove line number assumption from DefaultSharedDomain helper
Remove line number assumption from DefaultSharedDomain helper

- The DefaultSharedDomain integration test helper was updated in
  f39b70fae10cdcb9db780f6f7d7ce76e3b7d543b to no longer return the first
  domain
- This change introduced an assumption about the output of `cf domains`
  that is not valid for the v6 version of the command.

cloudfoundry/cli,cloudfoundry/cli,cloudfoundry/cli