prompt for host key confirmation instead of convoluted check

import sys

from paramiko import MissingHostKeyPolicy, SSHException

from paramiko import AutoAddPolicy, MissingHostKeyPolicy, SSHException, WarningPolicy

from invoke import Argument, Collection

from fabric import Connection

from fabric_tpa import host, install

from fabric_tpa.ui import VerboseProgram, hash_digest_hex

from fabric_tpa.ui import VerboseProgram, hash_digest_hex, no_yes

# suggestd in upstream paramiko: https://github.com/paramiko/paramiko/pull/2503

class PromptAddPolicy(MissingHostKeyPolicy):

    """

    Policy for automatically adding the hostname and new host key to the

    local `.HostKeys` object, and saving it.  This is used by `.SSHClient`.

    """

    def missing_host_key(self, client, hostname, key):

        WarningPolicy.missing_host_key(self, client, hostname, key)

        if not no_yes("Are you sure you want to continue connecting?"):

            raise SSHException(

                "Server {!r} not found in known_hosts".format(hostname)

            )

        AutoAddPolicy.missing_host_key(self, client, hostname, key)

class MatchingHostKeyPolicy(MissingHostKeyPolicy):

        if hash in self.fingerprints_md5_colons:

            logging.info('accepting fingerprint %s for host %s', hash, hostname)

        else:

            raise SSHException(

                "Server {!r} key {} not in trusted fingerprints: {!r}".format(

                    hostname,

                    hash,

                    self.fingerprints_md5_colons

                )

            )

            PromptAddPolicy.missing_host_key(self, client, hostname, key)

# this custom program (and, in fact, this entire script) is required