Several minor changes to the openstack manifest (take two):

- Use generic::mysql::packages::server rather than locally defining
  the mysql-server package.

- Use generic::mysql::packages::client rather than locally defining
  the mysql-client package.

- Use webserver:php5 rather than locally gathering up all
  the packages we would need for equivalent functionality.

- Add "if !defined" wrappers around several declarations in
  order to prevent conflicts with mediawiki_singlenode

- Make sure /a gets created before we try to use /a/backup

Change-Id: I4416a50a22cad35b852e39903cd8f7b2ceee37ae