Deprecate openstack server migrate --host option

Per the discussion at the Train Forum [1] this deprecates
the problematic --live option on the server migrate command
which, depending on the compute API version used, forcefully
bypasses the scheduler and also does not allow you to live
migrate a server and let the scheduler pick a host.

The --live option is replaced here with two new options:

* --live-migration: this simply tells the command you want to
  perform a live rather than cold migration; if specified with
  --live the --live-migration option takes priority.

* --host: when specified, this will request a target host for
  the live migration and will be validated by the scheduler;
  if not specified, the scheduler will pick a host. This option
  is mutually exclusive with --live.

We can build on the --host option by supporting cold migrations
with a specified host when using compute API version 2.56 or
greater but that will come in a separate change.

If the --live option is ever used we log a warning.

Note there are several related changes for this issue:

- https://review.openstack.org/#/c/628334/
- https://review.openstack.org/#/c/626949/
- https://review.openstack.org/#/c/627801/
- https://review.openstack.org/#/c/589012/
- https://review.openstack.org/#/c/460059/

This change allows us to deprecate the --live option and provide
a replacement which is backward compatible without having to use
something potentially error-prone like nargs='?'.

Closes-Bug: #1411190

[1] https://etherpad.openstack.org/p/DEN-osc-compute-api-gaps

Change-Id: I95d3d588e4abeb6848bdccf6915f7b5da40b5d4f

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