VMware: Handle concurrent inventory folder create

VMware: Handle concurrent inventory folder create

Cinder volumes are organized under vCenter inventory folder
with name given by config option vmware_volume_folder. During
volume creation, this folder is created only if it doesn't
exist. Concurrent volume create operations when there is no
such folder can result in a race condition-- some of the
create operations will fail due to already existing folder.
This patch fix this by ignoring the exception and returning a
reference to the existing folder.

Change-Id: Ic9371957bb76a032985363880fbebda523574305
Closes-Bug: #1436844