Make percona work with the rest of openstack in our POC env:

- percona listens on all ips, for now.
- don't install mysql-server package.
- remove default creds from percona/vars/defaults, for now,
  until we can get var overrides working as expected.
Merge pull request #5 from blueboxgroup/xtradb-poc

Make percona work with the rest of openstack in our POC env: