allowing zero as a valid value for a nagios interval
changing the seconds test to use #.to_i, as requseted by code review.  Adding an additional test to validate the interval is possible.
Use the nagios server download url attribute to download nagios source

This matches the expectation a user would have
Merge pull request #101 from tas50/COOK-2275

COOK-2275: Use the nagios server download url attribute to download nagios source
Merge pull request #89 from maplebed/COOK-2188

allowing zero as a valid value for a nagios interval