MDL-53945 mnet: Use moodle_url to provide string with url

Using moodle_url where possible is preferable to hardcoding uri's as
moodle_url is able to take care of alot of the formatting for us.

In this instance, it handles the $CFG->admin url path for us.

admin/mnet/peer_forms.php

@@ -61,7 +61,7 @@ class mnet_simple_host_form extends moodleform {
         if ($host = $DB->get_record('mnet_host', array('wwwroot' => $wwwroot))) {
             global $CFG;
             return array('wwwroot' => get_string('hostexists', 'mnet',
-                $CFG->wwwroot . '/' . $CFG->admin . '/mnet/peers.php?hostid=' . $host->id));
+                new moodle_url('/admin/mnet/peers.php', array('hostid' => $host->id))));
         }
         return array();
     }