Add mysql-mariadb-compat element

This allows distributions that have dropped the mysql aliases from
their mariadb packages to continue using the mysql element by
creating the necessary /var/run/mysqld directory.

At the moment this is only for RHEL7, but in the future it is
likely that Fedora will need the same thing because apparently this
is the approved direction for the mariadb package.

Change-Id: I2cc664e92b14db6bfdc2f33d0360001851e8ea1c
Add no-op cinder-tgt element

This will be required so tripleo-incubator change
I1e33af0cb78fa9477bc5a0b9b89b06b907b81fae can merge without
breaking CI.  This will need to merge, then the tripleo-incubator
change can merge, then the cinder refactor can merge.

Change-Id: I59091b047a70c1cf033733f4d1beaf6858574f94
Merge "Add mysql-mariadb-compat element"
Merge "Add no-op cinder-tgt element"