Improve tasks to make OpenStack configuration idempotent (no `shell`)

During #6 we [discussed the idempotence of configuring OpenStack](https://github.com/EGI-Foundation/ansible-packstack/pull/6#discussion_r178454241) using `shell` actions. I suggest that since these were interacting with a mysql server, the relevant `mysql-*` modules should be used, but @gwarf mentioned that this was not a good fit for what needs to happen there. Instead this should be done with the relevant `os_*`