Add ability to validate standalone .pp files from outside valid module directory

[sanfrancrisko]

Originally raised in PDK-711 :

Would like the ability to run an arbitrary puppet validate on any standalone .pp file and not be required to be in a module directory or be required to download and install the puppet agent.

Currently, when you try to do this, you get the following error:

joel:bin root# pdk validate puppet test.pp
\pdk (ERROR): This command must be run from inside a valid module (no metadata.json found).

This will work on arbitrary puppet files outside of a module but to run this arbitrarily, requires that you be working in an existing and valid module directory.

If this is a special case or could cause problems with usage elsewhere, I think it's acceptable to add this as an additional flag on the command line. Otherwise, maybe just change the existing error to a warning when the user tries to validate arbitrary puppet code outside of a valid module directory?