Kitchen test repositories are invalid for Ubuntu 16.04

[Lirt]

Hi, I was testing this formula with test-kitchen and ubuntu 16.04 today and found several issues with repositories.

First of all the repository for galera changed its structure. and we need to use these following URLs in tests.

http://releases.galeracluster.com/galera-3
http://releases.galeracluster.com/wsrep-5.6 or http://releases.galeracluster.com/wsrep-5.7

Second issue is that package python-mysqldb installs as dependency libmysqlclient20 and mysql-common 5.7 which conflicts with wsrep-5.6 and all other packages with version 5.6.

Third issue was missing key_ids for test repositories.

Edit: I created new issue for conflict mentioned above (Second issue) with mysql-common 5.7 https://github.com/salt-formulas/salt-formula-galera/issues/35

[Lirt]

As you can see in log lines from 1133to 1152 - https://travis-ci.org/salt-formulas/salt-formula-galera/jobs/294505874#L1133. There is an issue with installing mysql-wsrep-5.6. This is issue I mentioned here -- https://github.com/salt-formulas/salt-formula-galera/issues/35 and bumping repository to 5.7 fixes the issue.

[Lirt]

Thank you for merge