Use backwards compatible flag for Promoter

This commit will make the Promoter skip commits that have been promoted
already.
It also changes the --points-at flag to --contains to achieve the same
effect but be tolerant on Git versions
Cleans up stale vCenter folders and VMs

 * Empties VM Folder for Datacenter managed object
   before each BATs run

Signed-off-by: Brian Cunnie <cunnie@pivotallabs.com>
Rescues exceptions when cleaning up after BATs

 * This fixes a corner-case where the simultaneous
   execution of Centos & Ubuntu BATs will cause
   one to fail (both will see the folders, only the
   lucky one will delete it)

Signed-off-by: Brian Cunnie <cunnie@pivotallabs.com>