Pin ironic-python-agent in ussuri
Pin ironic-python-agent in ussuri

After [1] ipa package no longer is building on python2.7,
and there seems no plan to get it supported so let's pin
it until RDO moves to CentOS8.

[1] https://review.opendev.org/#/c/696378/

Change-Id: Ie218ec87fcf202c1b4d41ace2d91a5a8adf375f1

javierpena/rdoinfo,redhat-openstack/rdoinfo