Document that nova-network and VDS port group must be ephemeral

Add content clarifying that the port binding for the br100 port
group must be configured as ephemeral in vCenter. Included link
to a KB article that discusses port bindings.

Change-Id: I5bc2dcad56a8e6e54bb4a7476c89cbcaff6970aa
Closes-Bug: #1270306