Add NodeAdminUserData interface for "heat-admin" user

Add NodeAdminUserData interface for "heat-admin" user

Reinstates the heat-admin user via template user-data, which
replaces the previous boothook injected user provided by the
(deprecated now removed) heat instance_user option.

This has some advantages over the heat.conf option, e.g it allows
for much easier customzation of the user configuration (additional
SSH keys, adding groups etc), and also in future if we support
deploying more than one overcloud you could specify a different
user per deployment.

Co-Authored-By: Dan Prince <dprince@redhat.com>
Change-Id: I2235b9690c01542d8a28ec1c1a4607de751aea29
Closes-Bug: #1229849