Bind REST API to localhost by default

Binding to localhost by default is more secure than binding to all
interfaces.

Documentation/Getting-Started/Configuration-Guide.md

@@ -598,8 +598,7 @@ configuration file.
 #### `admin_host`
 
 The network interface where the HTTP admin interface listens on. The default
-value is the IPv6 address `::` which listens on all available network
-interfaces.
+value is the IPv4 address `127.0.0.1` which only listens for local connections.
 
 #### `admin_port`
 

include/maxscale/config.h

@@ -32,7 +32,7 @@ MXS_BEGIN_DECLS
 
 /** Default port where the REST API listens */
 #define DEFAULT_ADMIN_HTTP_PORT 8989
-#define DEFAULT_ADMIN_HOST      "::"
+#define DEFAULT_ADMIN_HOST      "127.0.0.1"
 
 #define RELEASE_STR_LENGTH      256
 #define SYSNAME_LEN             256

maxscale-system-test/cnf/maxscale.cnf.template.maxctrl

@@ -2,6 +2,7 @@
 threads=4
 admin_auth=false
 log_info=1
+admin_host=::
 
 [MySQL Monitor]
 type=monitor