(PUP-2732) Use less restrictive exec user validation

Without this patch Puppet agent validates the user attribute of exec ressource that way, that it's fully unsupported if running without root user. Doing so, the following error message is printed and execution aborted:
"Only root can execute commands as other users". This patch ignores a given user to run the exec as if it's the same as the user actually running the Puppet agent process.
See https://tickets.puppetlabs.com/browse/PUP-2732 for more information.