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 commit 'refs/changes/03/127603/4' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/11/129211/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch