Asterisk app: add IAX2 peer graphs (#11078)

* Asterisk app: add IAX2 peer graphs

* It's an entirely new graph and will not break existing graphs.
* The asterisk extend script must be updated to provide the appropriate poller data.
* The PR for librenms-agent/snmp/asterisk is here: https://github.com/librenms/librenms-agent/pull/274

* code climate, round 1

* clarified unit text

* code climate, round 2

* changes needed to properly generate test data

* linux_asterisk-v1 test data

includes/html/pages/apps.inc.php

@@ -306,6 +306,7 @@ $graphs['asterisk'] = array(
     'calls',
     'channels',
     'sip',
+    'iax2',
 );
 echo '<div class="panel panel-default">';
 echo '<div class="panel-heading">';