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.

test/shell/t6.sh

@@ -7,7 +7,7 @@ if env | grep -q -e '^TRAVIS=true$'; then
 fi
 
 # run till completion
-timeout --kill-after=20s 15s ./mgmt run --file t6.yaml --no-watch --tmp-prefix &
+timeout --kill-after=20s 15s ./mgmt run --yaml t6.yaml --no-watch --tmp-prefix &
 pid=$!
 sleep 1s	# let it converge
 test -e /tmp/mgmt/f1