Relax valadation for newer Periodic interval options

[micah]

Since apt version 1.5~beta2, APT::Periodic can take intervals in seconds, minutes and hours by adding suffixes (s, m, or h), so you can get things like:

APT::Periodic::Unattended-Upgrade "3h";

Unfortunately, the validation in the puppet module requires that this be either an integer or the string 'always', so it is not possible to pass any of these alternative intervals.

It would be nice if the validation was relaxed, so these can be passed!