When creating server group, the '--policy' option is required, but
valid choice is 'affinity' or 'anti-affinity', adding multiple
policies will produce an error. This patch add choices and default
value for option '--policy', and remove the former help message.
Change-Id: I312680af384363b3e9a7de9aa1f0946643e193e2
Closes-Bug: #1662771