Test: Allow to test different roles

While doing a make test, it could be interesting to test other roles
than mysql.

Let's use TEST_ROLE variable to define which role you are interested in.

    make test TEST_ROLE=openstack-compute

Enjoy,
packages: Cleaning all cache on yum based system

cleaning packages was not enough since in let more than 400MB of useless
stuff in /var/cache

So let's clean everything to make images smaller.