Add admin_pass as a property

This allows users to add the admin password as one of the template's
parameters so that nova servers are created using it.

Co-Authored-By: Richard Lee <rblee88@gmail.com>
Implements: blueprint custom-adminpass (partial)
Change-Id: I800601f95682ad23ce2e34c7168aec7ada8e03a5
Merge "Add admin_pass as a property"