Add scheduler hints

This blueprint adds a new attribute "scheduler_hints" to
create_instance(). This feature enables users to make some
advices (like affinity/anti-affinity) for nova scheduler.
For example, users can pass hints to schedule master and
slave nodes into different compute node.

# create server-group with "anti-affinity" policy
% nova server-group-create foo anti-affinity
+--------------------------------------+------+--------------------+---------+----------+
| Id | Name | Policies | Members | Metadata |
+--------------------------------------+------+--------------------+---------+----------+
| ef1e5347-acbf-4123-8445-858e61d8f48f | foo | [u'anti-affinity'] | [] | {} |
+--------------------------------------+------+--------------------+---------+----------+

# call Trove API with scheduler_hints to specify server-group
body = {
    "instance": {
        "volume": {"size": 5},
        "flavorRef": "2",
        "name": "foo",
        "datastore": {"version": "5.6", "type": "mysql"},
        "replica_count": 1,
        "scheduler_hints": {"group": "ef1e5347-acbf-4123-8445-858e61d8f48f"}
    }
}
print t.instances._create("/instances", body, "instance")