Shahzeb/asterisk mixin (#751)

* Add asterisk-mixin

* Update Lint Exclusions

* Update fixes to dashboard based on linter

* Add logs dashboard
Update Asterisk overview dashboard

* Update mixin Readme and update dashboards

* Add alerts.yaml - Update dashboards

* Dashboard and alerts update

* Updates based on rgeyer's review

* Updating lint exceptions file

asterisk-mixin/.lint

@@ -1,5 +1,7 @@
 exclusions:
-  template-datasource-rule:
-    reason: "templated datasource variable named 'prometheus_datasource', should be names 'datasource'"
   template-job-rule:
-    reason: "ob template should use datasource '$datasource'"
+    reason: "Prometheus datasource variable is being named as prometheus_datasource now while linter expects 'datasource'"
+  panel-datasource-rule:
+    reason: "Loki datasource variable is being named as loki_datasource now while linter expects 'datasource'"
+  template-datasource-rule:
+    reason: "Based on new conveention we are using variable names prometheus_datasource and loki_datasource where as linter expects 'datasource'"