Updating check validation for unpublished checks.

Unpublished checks (configured with the 'publish' attribute
as false) are not scheduled to run on any interval.  Hence,
these checks do not require a valid interval to run.  This
change updates the check validation to take this into consideration.
Merge pull request #532 from nstielau/unpublished_check_validation

Updating check validation for unpublished checks.