Add support for VMDK driver

The patch adds support for VMware vmdk cinder driver. It also
supports configuring the driver for multi-backend scenarios.

Note: Removed vmdk backend support from parent commit due to lack
of multi-backend puppet support in Havana.

implements blueprint support-vmdk-driver

Change-Id: I3f50f9974100ceab2a991f692022dd918f2e4445
(cherry picked from commit 14be6892554ab1c4df6144a0c782bad6b8c9cfec)
Merge "Add support for VMDK driver" into stable/havana