Do some Train TODOs

Make an early start on the TODOs for the Train cycle.

1. Remove the task that removes the vitrage_collector container, which
   was added in the Stein cycle to clean up this container which is no
   longer deployed.

2. Remove globals.yml configuration in CI to disable Heat for upgrade
   jobs. Heat is now enabled in the previous release (Stein).

3. Remove the deprecated variable cinder_iscsi_helper, which was renamed
   to cinder_target_helper in Stein.

Change-Id: I774bf395e0bdd4db9c20c6289a22cf059fa42e1a