Don't check for a package priority to be set when removing an APT preferences snippet.

The problem I'm facing is that the sanity checks prevent one from
using a simple:

  apt::preferences_snippet { "bla": ensure => absent }

So, first set a default value for the `priority' parameter, so
that it's not required anymore. Second, add a sanity check to
error out when priority is not set, to get the safe old behaviour.

Then, wrap all sanity checks about arguments within
a "if $ensure == 'present'" block.