(PUP-1174) enable the purging of authorized ssh keys

Purging authorized ssh keys is difficult because there is no way know
where key files may be located. Users commented that for most it should
be sufficient to clean the keys from users that puppet manages.
John Bollinger suggested a design that's based on a new parameter to the
user type. There are two ways to use it:

user { "name": purge_ssh_keys => true }

to make puppet purge from the default ~/.ssh/authorized_keys file, or

user { "name": purge_ssh_keys => [ "/etc/keyfiles/name" ] }

to specify one or more files in non-standard locations.

The implementation is similar to the file type's purging code. The user
type now has an eval_trace method that searches the specified files for keys
and adds any unmanaged keys to the catalog with ensure => absent.