Select mirror when diffing JJB XML files.

* .../jenkins_job_builder/config/job-builder.yaml:
* .../jenkins_job_builder/config/openstack-ci-puppet.yaml:
Run select-mirror.sh before running the compare scripts. This ensures
that dependencies not in the OpenStack mirror can be downloaded. Without
it pip may be configured to only use the OpenStack mirror.

Change-Id: Ie1ce67deb78e4bdfe20884bb6bda0fbb57db2f0e
Merge "Select mirror when diffing JJB XML files."