Server secgroups and network/port together invalid

Nova silently ignores security_groups properties for a server if the
server has a 'networks' property that references a neutron port, the
intent being that security groups are assigned to the port(s).

Adds note to Server properties to this effect.
Validation fails for a Server if 'security_groups' is defined AND
the server has neutron Ports assigned.

Change-Id: I883761a0bb0e73a3fcc7c362d8e6dc5a099806e0
Closes-Bug: #1270313
Merge "Server secgroups and network/port together invalid"