NSX Plugin note.

AMENDED - removed reference to Neutron per Andreas Jaeger's comment.

Adds note about NSX plugin as only validated plugin for vSphere.

Change-Id: Id0fa45ae1160e70359ed534b08333128806ff905
Closes-Bug: #1255772
(cherry picked from commit 42ca862caf9059e6c8db1763874199148ace4216)
Added missing dependency in glance install in ubuntu.

Closes-Bug: #1258030

Change-Id: I14b8d1685da91d669ca5d33fb9b64614f8877de0
backport: havana
(cherry picked from commit b74b68dfe0cff76bfa84065ccb53b08dc5706ddf)
Merge "Added missing dependency in glance install in ubuntu." into stable/havana
Merge "NSX Plugin note." into stable/havana