Fix mariadb & galera install order

* We were installing mariadb/galera as part of the galera play to ensure
  we could control the startup and preseed.
* This changes the play back to do this properly, instead of installing
  it as part of the "common" play.
* This leaves the packages to be installed in the common repo_packages
  file

Fixes-Bug: #222
Merge pull request #224 from andymcc/galera_fix

Fix mariadb & galera install order