Switch from subprocess to keystoneclient

To slam the coffin lid shut on using subprocess in nodes, move the
keystone service listing to making use of keystoneclient. As an added
bonus, the method making use of it is now actually tested.

Change-Id: Ifec5fe18c506ecf8647db9e42b71ade0a1dc4e73
Merge "Switch from subprocess to keystoneclient"