document VMDK driver support for Cinder block storage

- mention that starting with Fuel 5.1 we support VMDK driver for Cinder
- explicitly state that user has to change Cinder backend after
  finishing environment creation with vCenter as hypervisor option
- update screenshot where user selects storage backends for Cinder and
  Glance, right now there is no choice available, becase currently Ceph
  cannot be used with vCenter
- add screenshot showing where user can change Cinder storage backend
- correct Cinder definition in Terminology reference, that *VMDK*
  driver enables vCenter datastores support, not *vCenter* driver
- remove limitation in Planning guide that tells that VMDK driver is not
  supported, which is not true anymore
- add limitation in Planning guide that the only supported backend for
  Cinder is VMDK driver

Implements: blueprint vcenter-documentation-update-fuel-5.1
Change-Id: Idc2412f2d897d76e68c968bb77b84e350af07cd1
Merge "document VMDK driver support for Cinder block storage"