Configure sql_mode options

In order to get Gerrit working with Galera we need to modify
sql_mode options. With default options following SQL query
fails with "Invalid default value for 'added_on'":

CREATE TABLE account_group_by_id_aud (
  added_by INT DEFAULT 0 NOT NULL,
  removed_by INT,
  removed_on TIMESTAMP NULL DEFAULT NULL,
  group_id INT DEFAULT 0 NOT NULL,
  include_uuid VARCHAR(255) BINARY DEFAULT '' NOT NULL,
  added_on TIMESTAMP NOT NULL
  ,PRIMARY KEY(group_id,include_uuid,added_on)
)

Change-Id: Ic44192712512bb824890285b27eb353e4a37f4f4

service/files/defaults.yaml

@@ -9,6 +9,7 @@ configs:
     xtrabackup_password: "password"
     monitor_password: "password"
     gcache_size: "1G"
+    sql_mode: null
     cluster_size: 3
     force_bootstrap:
       enabled: false

service/files/my.cnf.j2

@@ -25,6 +25,10 @@ innodb_write_io_threads = 8
 
 open_files_limit = 102400
 
+{% if percona.sql_mode %}
+sql_mode = "{{ percona.sql_mode }}"
+{% endif -%}
+
 wsrep_slave_threads = 4
 wsrep_cluster_address = gcomm://
 wsrep_provider = /usr/lib/galera3/libgalera_smm.so