Fail validation when security groups and interfaces conflict

The 'SecurityGroups' property of the instance is not taken into account
by Nova when the 'NetworkInterfaces' property is also defined. Instead the
security groups should be applied to the network interface resource(s).

This issue has already raised a couple of invalid bugs on Launchpad.

Change-Id: I5ae2ba356518d391893c2a479c5952b13b6e8d55
Merge "Fail validation when security groups and interfaces conflict"