'policy' should be required in server_group_create

The 'policies' param is a required param in nova api now.
So, we also need to change it in cli.

After the modification, the 'policy' param will be required now,
and the old '--policy' style will still be available.

Change-Id: I97921eef0f635c8d50a04bbdaf3ef4f1bfaa3e9d
Closes-Bug: #1323909
Merge "'policy' should be required in server_group_create"