roles/mariadb: Add plays for creating WordPress db/users

Relies on the host having a dict with appropriate values defined,
for example:

  wordpress_blogs:
    - site_name:            blah.com
      wordpress_version:    3.9.2
      wordpress_db_user:    db_user
      wordpress_db_name:    db_name
      wordpress_db_pass:    V9&XvvKu1hYl

Signed-off-by: Alan Orth <alan.orth@gmail.com>