Support max_user_connections in database_user

Here's code to support [max_user_connections](http://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html#sysvar_max_user_connections) on the account level.

This is my first venture into rspec, so I can't promise that the tests are any good.

Unfortunately it cannot be added during user creation without changing `CREATE USER` to a `GRANT` statement. I can do this if desirable.