The hard-coded choices for the server group policy make it impossible
to create a server group with the soft-affinity or
soft-anti-affinity policy rules which were added in compute API
microversion 2.15. This removes the hard-coded choices so that the
policy is restricted on the server side rather than the client side.
Change-Id: Ib3dc39422ac1015872d56ae2fdeddf0f29613494
Closes-Bug: #1732938