Don't cache check for cdrom source in the apt provider.

Ruby 1.9 warned about access to a class variable in providers, because their
semantics are almost never what users expect.

However, the use of a class variable in the apt provider was very strange: it
wanted to check if a cdrom source was specified, and fail if it was present,
to avoid a situation where apt would prompt for a CD to be inserted and hang
forever.

Since that is just as much a problem on the second package install as on the
first, it makes much more sense to perform this sanity check and fail before
every install - and the overall cost is relatively low.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>