CP-17414: Add AD feature check to host.enable_external_auth
CP-17414: Add AD feature check to host.enable_external_auth

This indirectly applies to pool.enabled_external_auth as well.  The feature
check is done only if `auth_type = "AD"`.

The check is in Message_forwarding, so that it is done on the master, for
efficiency.

Signed-off-by: Rob Hoes <8783b3376fb07d39edffc8d7da68a6de3874a7e6@citrix.com>

jjd27/xen-api,jjd27/xen-api,jjd27/xen-api,jjd27/xen-api,jjd27/xen-api,jjd27/xen-api