Use root pass for galera init script

The galera init script uses the debian-sys-maint account to shutdown
mysql. However this password cannot be preseeded, and is randomly
generated on install. Also it is set in the database so must be updated
when replication has been achieved.

This patch implements an alternative: Use the mysql root password for
init script operations (shutdown). This password is preseedable and will
work at any state of replication.

This is no less secure than previous state as we already write root galera
credentials to /root/.my.cnf (0600) and /etc/mysql/debian.cnf is also 0600.

Related: #411

Conflicts:
	rpc_deployment/inventory/group_vars/galera.yml
Use local socket for mysql client connection

This affects mysql client as well as mysqladmin, which is used in status
checks in the init script. It is important that the init script can
succeed regardless of LB state. (Prior to this commit, LB IP was used as
the host to connect to).

Related: #413
Specialize galera client config

For reliable operation, the mysql client configuration must point to the
local instance, within galera containers. However this prevents the
mysql client from working on other constainers (eg utility).

This patch defaults the mysql connection to the VIP, then overrides that
to 127.0.0.1 for galera containers.

Related: #413
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
Merge pull request #491 from hughsaunders/galera_backports

Galera backports