[NRRPLT-3902] Various fixes after Lugano vizcluster testing.

This patch resolves the remaining issues with distributed Nest launching on the
Lugano vizcluster. It includes fixes to:

- pass the music working directory path as an environment variable rather than
  using MUSIC file configuration, as the CLE seems to change working directory
  during the launch of an expeirment and the proxy.xml file was inaccessible

- fix launch success/failure to wait for a non progress bar status message from
  the CLE, this is because the status bar messages are always marked as 'done'
  even in the event of a failure, so it was not a correct way to check success

- start gzbridge on the backend VM by waiting for Gazebo to be started on the
  remote nodes, this is because the vizcluster node cannot start gzbridge and
  would not be reachable by the frontend even if it could

Change-Id: Id182da74b6e01e748003bc8d7694fc83fe572336