(PUP-2737) move regex from ssh_authorized_key provider to the type

Defining the regular expression for the ParsedFile based provider of
the ssh_authorized_key type on the type level rather that directly in
the provider code, it becomes easily possible to reuse it in other types.
(PUP-2737) use the general regex to match ssh_authorized_keys for purging

In order to generate ssh_authorized_key resource for purging, the user
type needs its own parsing logic for ssh_authorized_keys files, as it
cannot directly emply a provider of ssh_authorized_key. This was originally
implemented in a rather half-assed way and worked only for keys without
spaces in the comment string (which Puppet uses as the resource title).

To fix this, unify the parsing logic by using the same regular expression
with which the ssh_authorized_key provider extracts the respective fields
from the keyfiles.
(PUP-2737) fix an error in the user type unit test for ssh key purging

After fixing PUP-2737, it became apparent that there was an error in the
expectations agains the fixture for the user type unit tests. A key
from the fixture did contain spaces in its comment, but the unit test
actually failed to expect the correct value.

Now that the full resource title is retrieved, make a slight adjustment
so that the test keeps working and uses a more appropriate name for the
key.
Merge branch 'pull-2753'

* pull-2753:
  (PUP-2737) fix an error in the user type unit test for ssh key purging
  (PUP-2737) use the general regex to match ssh_authorized_keys for purging
  (PUP-2737) move regex from ssh_authorized_key provider to the type

This closes GH-2753.