Don't deprecate maint mode updates via node-update

As discussed in the weekly meeting[1], because there's no way to indicate
to the users of our API that maintenance mode updates via node-update will
no longer be acceptable, we will continue to support both endpoints for v1.

[1] starting at 17:13:45,
http://eavesdrop.openstack.org/meetings/ironic/2014/ironic.2014-12-01-17.02.log.txt

Change-Id: Ic76e0ca6c9747f17887742032ee3a49fbd19cf9d
Merge "Don't deprecate maint mode updates via node-update"