Add user's SSH public key to their LDAP entry.

def add(config, username, filename):
        """Add user's SSH public key to their LDAP entry."""
        try:
            client = Client()
            client.prepare_connection()
            user_api = UserApi(client)
            key_api = API(client)
            key_api.add(username, user_api, filename)
        except (ldap3.core.exceptions.LDAPNoSuchAttributeResult,
                ldap_tools.exceptions.InvalidResult,
                ldap3.core.exceptions.LDAPAttributeOrValueExistsResult
                ) as err:  # pragma: no cover
            print('{}: {}'.format(type(err), err.args[0]))
        except Exception as err:  # pragma: no cover
            raise err from None