VMware: Fix volume detach failure

The VMwareVCDriver method "detach_volume" is missing the "encryption"
argument and thus does not conform to the driver API. Adding the missing
argument solves TypeError being thrown when detaching a volume.

Change-Id: Ib785c2b2cf4c25deed272f8e3f0f47abe6139d1c
Closes-Bug: #1226826
Merge "VMware: Fix volume detach failure"