VMware: ephemeral disk support

This patch ensures that the driver honors the following for
ephemeral disks:
 1. the flavor definition
 2. the block device mappings

The driver will now create an ephemeral disk(s) if necessary.

The ephemeral disks will be of a 'thin' disk type.

Closes-bug: #1369254

This is part of blueprint vmware-ephemeral-disk-support

DocImpact
  - Indicate that ephemeral disks are now supported by the driver

Change-Id: Icb1796e58126a1e03613946b0f5a917d6eb4708e
Merge "VMware: ephemeral disk support"