(PUP-1413) Add 'touch' command to Augeas provider

The touch command works like its POSIX namesake, only creating the node in
the Augeas tree when it doesn't already exist, and creating it with a NULL
value.  The behaviour is copied from augtool (augrun).
Merge pull request #3377 from domcleal/tickets/master/pup-1413-augeas-touch

(PUP-1413) Add 'touch' command to Augeas provider