VMware: Added doc for VMware VMDK cinder driver

Added document describing configuration of the driver and supported operations
when using it for cinder volume management.

Updated volume section in doc/common/section_vmware.xml.

Change-Id: I1181f8c834c916ef241a5abdeb48ab85b778d8e7
Implements: blueprint vmware-vmdk-cinder-driver
(cherry picked from commit 280d84e86602fd5ee6bfb50c31c3e61987e461f1)