rbd: Revert -blockdev and -drive parameter auth-supported

This reverts half of commit 0a55679.  We're having second thoughts on
the QAPI schema (and thus the external interface), and haven't reached
consensus, yet.  Issues include:

* The implementation uses deprecated rados_conf_set() key
  "auth_supported".  No biggie.

* The implementation makes -drive silently ignore invalid parameters
  "auth" and "auth-supported.*.X" where X isn't "auth".  Fixable (in
  fact I'm going to fix similar bugs around parameter server), so
  again no biggie.

* BlockdevOptionsRbd member @password-secret applies only to
  authentication method cephx.  Should it be a variant member of
  RbdAuthMethod?

* BlockdevOptionsRbd member @user could apply to both methods cephx
  and none, but I'm not sure it's actually used with none.  If it
  isn't, should it be a variant member of RbdAuthMethod?

* The client offers a *set* of authentication methods, not a list.
  Should the methods be optional members of BlockdevOptionsRbd instead
  of members of list @auth-supported?  The latter begs the question
  what multiple entries for the same method mean.  Trivial question
  now that RbdAuthMethod contains nothing but @type, but less so when
  RbdAuthMethod acquires other members, such the ones discussed above.

* How BlockdevOptionsRbd member @auth-supported interacts with
  settings from a configuration file specified with @conf is
  undocumented.  I suspect it's untested, too.

Let's avoid painting ourselves into a corner now, and revert the
feature for 2.9.

Note that users can still configure authentication methods with a
configuration file.  They probably do that anyway if they use Ceph
outside QEMU as well.

Further note that this doesn't affect use of key "auth-supported" in
-drive file=rbd:...:key=value.

qemu_rbd_array_opts()'s parameter @type now must be RBD_MON_HOST,
which is silly.  This will be cleaned up shortly.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 1490691368-32099-9-git-send-email-armbru@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
