heat_keystoneclient: abstract admin_client to a property

To make the connection to keystone with admin credentials reusable
store the admin client object at the class level rather than local
to create_trust_context.  This will avoid the overhead of
getting two authenticated clients when other admin operations are
necessary.

Change-Id: Iaa043b829778f71d03e64a78ccb3f51eadfb3fba
blueprint: instance-users
Merge "heat_keystoneclient: abstract admin_client to a property"