Increase galera startup timeout

The mysql init script pings mysql once per second for thirty seconds,
then reports failure, even though mysql may eventually start
succesfully. We run into this problem sometimes as galera startup can
take longer than standard mysql startup, especially when init or state
transfer is required.

The complication is that the init script does not source a defaults
file, so it is difficult to override the timeout.

In this commit, I have brought the init script into the role, and added
a single line to it, which sources /etc/mysql/default. I have also added
/etc/mysql/default and set a long timeout for MYSQL_DEFAULT_TIMEOUT
(1800 seconds).

Related: #413