dn: support conversion from python-cryptography Name
The upcoming change to using python-cryptography for certificate
process will require a way to convert
``cryptography.x509.name.Name`` values to ``ipapython.dn.DN``.
Update the ``DN`` constructor to accept a ``Name``.
Part of: https://fedorahosted.org/freeipa/ticket/6398
Reviewed-By: Jan Cholasta <jcholast@redhat.com>
Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>