Use UNIX socket instead of TCP socket

In certain use cases, you may be listening on a different interface
and it is not possible to listen to multiple interfaces with MySQL.

This changes the behaviour to connect via the local socket, as
Galera treats 'localhost' as a connection via the UNIX socket and
'127.0.0.1' as using the TCP socket.

This will allow using Galera on a specific interface while still
letting the bootstrap process talk to the local machine via the
socket which is still available.

Change-Id: Ib46f91e0a6becc934984d372bc51fc2aa77a0c95

templates/client.my.cnf.j2

@@ -2,7 +2,7 @@
 
 [client]
 host={% if inventory_hostname in galera_cluster_members %}
-127.0.0.1
+localhost
 {% else %}
 {{ galera_address }}
 {% endif %}

templates/releasenotes/notes/use-unix-socket-578a4a32490c5e22.yaml

@@ -0,0 +1,4 @@
+---
+upgrade:
+  - The role now uses the UNIX sockets to communicate to the server instead
+    of TCP.