compute: Move server migrations commands to their own file

The 'openstackclient.compute.v2.server' module is getting rather large.
The server migration commands don't need to be in there. Move them.

Change-Id: I8b2600cfd9f8e37d3093c52c7222d85e84e7fc89
Signed-off-by: Stephen Finucane <sfinucan@redhat.com>

Author: stephenfin