Use socket only if use_socket parameter is set to True

server.backend.socket is currently defined
in map.jinja which forces to use socket all the time.

Related-PROD: https://mirantis.jira.com/browse/PROD-16133

Change-Id: I7e1b3011926305d7e9164e4b6772ddf197ffd85a
diff --git a/powerdns/files/backends/mysql.conf b/powerdns/files/backends/mysql.conf
index f064469..cc54238 100644
--- a/powerdns/files/backends/mysql.conf
+++ b/powerdns/files/backends/mysql.conf
@@ -5,7 +5,7 @@
 launch+=gmysql
 
 # gmysql parameters
-{%- if server.backend.socket is defined %}
+{%- if server.backend.get('use_socket', False) == True %}
 gmysql-socket={{ server.backend.socket }}
 {%- else %}
 gmysql-host={{ server.backend.host }}