Rationalize configuration for percona/galera, add generic helpers for parsing mysql configuration options, use mysqlhelper for creation of SST user

templates/my.cnf

@@ -1,3 +1,4 @@
+# Juju managed file - don't change as charm will overwrite your changed!
 [mysqld]
 
 datadir=/var/lib/mysql
@@ -36,4 +37,17 @@ wsrep_cluster_name={{ cluster_name }}
 # Authentication for SST method
 wsrep_sst_auth="sstuser:{{ sst_password }}"
 
+{% if max_connections != -1 %}
+max_connections = {{ max_connections }}
+{% endif %}
+
+# Fine tuning
+key_buffer_size = {{ key_buffer }}
+table_cache = 512
+max_allowed_packet = 16M
+
+# InnoDB buffer should consume 100% of the bytes of the dataset size
+# query cache is not supported with Active/Active configuration
+innodb_buffer_pool_size = {{ dataset_bytes }}
+
 !includedir /etc/mysql/conf.d/