Updated to reflect new 6.0 feature

* Add support for vCenter+Glance feature
* Remove limitations on using this feature
* Add explanation into terminology issues

Change-Id: I1d4a1c397399c25f66804936fdb451095ccc1a75
Closes-Bug: 1379651
Implements: blueprint vsphere-glance-backend
Merge "Updated to reflect new 6.0 feature"