Allow install mariadb from RDO repository

Adds elements which allow installing mariadb from RDO repository.
It's similar to mariadb element but different repo with different
package names are used (also some installation steps might be removed
in future).

Common code for mariadb and mariadb-rdo is now in mariadb-common element.

Change-Id: I8566b03a24fff7984f323442d93249e1002a6ebe
Merge "Allow install mariadb from RDO repository"