Add configuration to allow the switch between dnf/yum or provide an
alternative package manager.
This reduces the overall size of the kolla images by 3 percents and
improves the build and push time by 5 percents according to my
Compare results with  and without  the patch applied.
This commit allows to configure yum settings via a yum.conf file that
is shipped in the base image and make it overridable with the
base_yum_conf jinja2 block.