lang: funcs: Add deploy package with readfile related functions

This adds a readfile function to actually access files from our deploy.
A fun side effect is that we can even access our own code! In general,
it's a good reminder that you should only run trusted code on your own
infrastructure. This also includes a fancy new test case.

lang/funcs/core/core.go

@@ -20,6 +20,7 @@ package core
 import (
 	// import so the funcs register
 	_ "github.com/purpleidea/mgmt/lang/funcs/core/datetime"
+	_ "github.com/purpleidea/mgmt/lang/funcs/core/deploy"
 	_ "github.com/purpleidea/mgmt/lang/funcs/core/example"
 	_ "github.com/purpleidea/mgmt/lang/funcs/core/example/nested"
 	_ "github.com/purpleidea/mgmt/lang/funcs/core/fmt"