python:key_credential_link: add descriptive methods

python:key_credential_link: add descriptive methods

In samba-tool we are going to want a KeyCredentialLinkDn to be able
to describe itself. We're adding the methods here because
`samba-tool user` and `samba-tool computer` will both want to use
them.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>