Add admin_pass as attribute and property to server

This allows users to specify an admin_pass parameter that will set the
root password of the server.

Implements: blueprint custom-adminpass (partial)
Change-Id: Ic11f1ddcd114631b92f87d5760ab7a6761ba40e4
Allow update of admin_pass on server

This makes it possible to issue an update that modifies the admin_pass
attribute of a server.  If the server does not support a root password
change, a ResourceFailure will be raised.

Implements: blueprint custom-adminpass
Change-Id: Ia2f596c93a2c448b98ac86b96385c8e9015435d4