[galera] Some master/slave tweaks

This adds some tweaks for galera master/slave setup

It also allows to customize the galera gcache size,
This value depends mainly of the database traffic.
But the default value of 128M is clearly to low for us, this means
if a down node comeback and the data that need to be transfered is
greater than 128MB a full sync if done instead of an incremental.

So we use 1G by default and that should
changed in case of a deployement with a big database workload.
[galera] use same value for galera and haproxy max conn

Use same value for galera and haproxy max connection

The extra connection will be dropped by haproxy instead of galera.