CI: Reduce unnecessary gate image builds

Due to regex matching of images, we are currently building quite a few
unnecessary images in CI. Images that can unexpectedly match:

* barbican-keystone-listener
* ironic-neutron-agent
* prometheus-haproxy-exporter
* prometheus-memcached-exporter

This change improves the situation by anchoring matches to the start of
the name. A full solution would be to specify every image required for
each job. This would be quite verbose and require maintenance.

Change-Id: I7387ccd0cb394c54c79bfba8b1c09fce7ee2973e