(PUP-2576) support purging of all cronjobs

Add a method to the Util::FileType class that lists all accessible
crontab files stored in the appropriate location. Using this method,
have the crontab provider add the present files as targets.

Consequently, the crontab provider can now generate resources for all
crontab entries in the system via its instances method. This is all
that is required to allow purging of all those that are not managed.
(PUP-2576) update crontab provider unit tests

Two tests need changing, because when running rspec as root, it should
actually be expected that there are cron resources, even though the
target user is absent or has no crontab. Change the test semantics so that
cron entries of other users are ignored. This fix was proposed by
John Bollinger.

Due to the parsing of system crontabs in addition to the fixture
(again, only if running as root), the target_object method will be
invoked more than once. We can no longer rely on expect(), because
we cannot know the number of invocations on any developer's machine.
Use stubs() instead and ignore the number of invocations. Technically,
this removes a check, but it's not an important aspect of the tests
in question.