Use service tenant id for Ironic agent swift auth

Config of ironic conductor currently references a non-existent variable.
This corrects it by obtaining and using the correct tenant id.

Change-Id: I4340f75b2b22a8fd3fd8dd5cb30d7e91bce4f654
Merge "Use service tenant id for Ironic agent swift auth"
Merge branch 'master' of https://github.com/openstack-dev/devstack