(maint) Exclude crontab integration specs on windows

Since windows doesn't have a functional crontab provider, it doesn't
make sense to run integration tests for cron on windows.
(maint) Refactor crontab filetype selection

The previous implementation of the crontab implementation type
determined the filetype to use at load time by calling out to Facter
before the type was loaded, which was both ugly and made it difficult to
test on alternate platforms. This commit overrides the `.filetype`
method to do the detection at runtime so the provider encapsulates all
its logic and the filetype can be stubbed out.
Merge branch 'maint-refactor_cron_filetype_selection'