Instance groups: validate policy configuration

Ensure that API does not allow conflicting policies to be configured
on an instance group. More specifically the user should not be allowed
to configured 'anti-affinity' and 'affinity' on the same instance group.

In addition to this a validation will also be done on the policy, that is,
if the policy is not supported then an exception will be raised.

This is part of blueprint instance-group-api-extension.

Change-Id: Id19c55cb60109819429f73e2b28efe7f15cc5194
Merge "Instance groups: validate policy configuration"