Stop using mysqlclient ssl flag

The ssl flag is deprecated and we get cronspam [0] warning us about
this. The docs [1] say we should use ssl-mode instead.

[0] WARNING: --ssl is deprecated and will be removed in a future version. Use --ssl-mode instead.
[1] https://dev.mysql.com/doc/refman/5.7/en/connection-options.html#option_general_ssl

Change-Id: I060bbfeaf1171dac50dcfcd2c62fcaa8956fb4e2

playbooks/roles/gerrit/templates/root.my.cnf.j2

@@ -2,4 +2,4 @@
 host={{ gerrit_mysql_host }}
 user=gerrit2
 password={{ gerrit_mysql_password }}
-ssl=false
+ssl-mode=DISABLED