openstack-full: Fix pinning of mariadb packages

Since the commit 42b6d58a82950711dec99dc908d0060491d1003f the pinning of
mariadb packages is not working because we use origin for it.
This commit modifies the pinning method from origin to release in order to not
hardcode repository address.

Signed-off-by: Dimitri Savineau <dimitri.savineau@enovance.com>
Merge pull request #106 from dsavineau/fix-mariadb-pinning

openstack-full: Fix pinning of mariadb packages