add pluginBundle support

A plugnBundle is meta plugin that has a set of dependent plugins
to enable.  This commit includes a plugin.json for a default
"core" bundle that enables all of the shipped panels and datasources.

pkg/models/plugin_bundle.go

@@ -0,0 +1,8 @@
+package models
+
+type PluginBundle struct {
+	Id      int64
+	Type    string
+	Org     int64
+	Enabled bool
+}