Detect invalid or missing properties for shellusers

Summary:
This test has been added to detect a typo in user properties,
like in D2115 "ssh-keys" instead of "ssh_keys".

Test Plan: Tested with D2215 bogus and fixed versions.

Reviewers: dereckson

Reviewed By: dereckson

Subscribers: alina-precup

Differential Revision: https://devcentral.nasqueron.org/D2125