Add admin_pass as attribute to CloudServer

This stores the admin password returned by nova() as an encrypted
property on the CloudServer to allow template authors to display them
as outputs of a stack.

Co-Authored-By: Anderson Mesquita <andersonvom@gmail.com>
Implements: blueprint custom-adminpass (partial)
Change-Id: Ic11f1ddcd114631b92f87d5760ab7a6761ba40e4
Merge "Add admin_pass as attribute to CloudServer"