def add_server_key(self, key):
self.server_key_dict[key.get_name()] = key<EOL>
Add a host key to the list of keys used for server mode.  When behaving
as a server, the host key is used to sign certain packets during the
SSH2 negotiation, so that the client can trust that we are who we say
we are.  Because this is used for signing, the key must contain private
key info, not just the public half.  Only one key of each type (RSA or
DSS) is kept.

:param .PKey key:
    the host key to add, usually an `.RSAKey` or `.DSSKey`.