gapi: Split out graph generation into a proper graph API

This is a monster patch that splits out the yaml and puppet based graph
generation and pushes them behind a common API. In addition alternate
pluggable GAPI's can be easily added! The important side benefit is that
you can now write a custom GAPI for embedding mgmt!

This also includes some slight clean ups that I didn't find it worth
splitting into separate patches.

gapi/gapi.go

@@ -0,0 +1,41 @@
+// Mgmt
+// Copyright (C) 2013-2016+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+// Package gapi defines the interface that graph API generators must meet.
+package gapi
+
+import (
+	"github.com/purpleidea/mgmt/etcd"
+	"github.com/purpleidea/mgmt/pgraph"
+)
+
+// Data is the set of input values passed into the GAPI structs via Init.
+type Data struct {
+	Hostname string // uuid for the host, required for GAPI
+	EmbdEtcd *etcd.EmbdEtcd
+	Noop     bool
+	NoWatch  bool
+	// NOTE: we can add more fields here if needed by GAPI endpoints
+}
+
+// GAPI is a Graph API that represents incoming graphs and change streams.
+type GAPI interface {
+	Init(Data) error               // initializes the GAPI and passes in useful data
+	Graph() (*pgraph.Graph, error) // returns the most recent pgraph
+	SwitchStream() chan error      // returns a stream of switch events
+	Close() error                  // shutdown the GAPI
+}