(MODULES-1582) File location placeholder

This makes the validate_cmd closer to the file implementation, and allows a user to
give a validate_cmd path where the filepath is not at the end of the string.

The example I found was with haproxy, which uses the following command for file validation:

`/usr/sbin/haproxy -f /etc/haproxy.cfg -c`

Currently this isn't possible to do with stdlib, so I made this PR to fix it! :dancer: