VerifyHostKeyDNS not used (lp:#1742105)

[Original report](https://bugs.launchpad.net/bugs/1742105) created by **ybovard (ybovard)**
```
I try to use the SSHFP resource records. Everything works well with ssh in
commandline but duplicity does not seem to take VerifyHostKeyDNS into
account:
# ls -l /root/.ssh/known*
-rw-r--r--. 1 root root 792 Jun 22  2017 /root/.ssh/known_hosts-old

# cat /etc/ssh/ssh_config
...
VerifyHostKeyDNS yes
...

# ssh user@myserver.mydomain.mytld
Last login: Tue Jan  9 10:43:03 2018 from xyz
$

# /bin/duplicity incremental --encrypt-key AAAA --exclude /tmp /
scp://user@myserver.mydomain.mytld//backup/comal.novalocal
Using archive dir: /root/.cache/duplicity/0ae77733cf532410912be8ca70d1f956
Using backup name: 0ae77733cf532410912be8ca70d1f956
GPG binary is gpg, version 2.0.22
Import of duplicity.backends.acdclibackend Succeeded
Import of duplicity.backends.azurebackend Succeeded
Import of duplicity.backends.b2backend Succeeded
Import of duplicity.backends.botobackend Succeeded
Import of duplicity.backends.cfbackend Succeeded
Import of duplicity.backends.dpbxbackend Failed: No module named dropbox
Import of duplicity.backends.gdocsbackend Succeeded
Import of duplicity.backends.giobackend Succeeded
Import of duplicity.backends.hsibackend Succeeded
Import of duplicity.backends.hubicbackend Succeeded
Import of duplicity.backends.imapbackend Succeeded
Import of duplicity.backends.lftpbackend Succeeded
Import of duplicity.backends.localbackend Succeeded
Import of duplicity.backends.mediafirebackend Succeeded
Import of duplicity.backends.megabackend Succeeded
Import of duplicity.backends.multibackend Succeeded
Import of duplicity.backends.ncftpbackend Succeeded
Import of duplicity.backends.onedrivebackend Succeeded
Import of duplicity.backends.par2backend Succeeded
Import of duplicity.backends.pydrivebackend Succeeded
Import of duplicity.backends.rsyncbackend Succeeded
Import of duplicity.backends.ssh_paramiko_backend Succeeded
Import of duplicity.backends.ssh_pexpect_backend Succeeded
Import of duplicity.backends.swiftbackend Succeeded
Import of duplicity.backends.sxbackend Succeeded
Import of duplicity.backends.tahoebackend Succeeded
Import of duplicity.backends.webdavbackend Succeeded
ssh: starting thread (client mode): 0x1d83e90L
ssh: Local version/idstring: SSH-2.0-paramiko_1.16.1
ssh: Remote version/idstring: SSH-2.0-OpenSSH_7.4
ssh: Connected (version 2.0, client OpenSSH_7.4)
ssh: kex algos:[u'curve25519-sha256', u'curve25519-sha256@libssh.org',
u'ecdh-sha2-nistp256', u'ecdh-sha2-nistp384', u'ecdh-sha2-nistp521',
u'diffie-hellman-group-exchange-sha256', u'diffie-hellman-group16-sha512',
u'diffie-hellman-group18-sha512', u'diffie-hellman-group-exchange-sha1',
u'diffie-hellman-group14-sha256', u'diffie-hellman-group14-sha1', u'diffie-
hellman-group1-sha1'] server key:[u'ssh-rsa', u'rsa-sha2-512', u'rsa-
sha2-256', u'ecdsa-sha2-nistp256', u'ssh-ed25519'] client
encrypt:[u'chacha20-poly1305@openssh.com', u'aes128-ctr', u'aes192-ctr',
u'aes256-ctr', u'aes128-gcm@openssh.com', u'aes256-gcm@openssh.com',
u'aes128-cbc', u'aes192-cbc', u'aes256-cbc', u'blowfish-cbc',
u'cast128-cbc', u'3des-cbc'] server
encrypt:[u'chacha20-poly1305@openssh.com', u'aes128-ctr', u'aes192-ctr',
u'aes256-ctr', u'aes128-gcm@openssh.com', u'aes256-gcm@openssh.com',
u'aes128-cbc', u'aes192-cbc', u'aes256-cbc', u'blowfish-cbc',
u'cast128-cbc', u'3des-cbc'] client mac:[u'umac-64-etm@openssh.com',
u'umac-128-etm@openssh.com', u'hmac-sha2-256-etm@openssh.com', u'hmac-
sha2-512-etm@openssh.com', u'hmac-sha1-etm@openssh.com',
u'umac-64@openssh.com', u'umac-128@openssh.com', u'hmac-sha2-256', u'hmac-
sha2-512', u'hmac-sha1'] server mac:[u'umac-64-etm@openssh.com',
u'umac-128-etm@openssh.com', u'hmac-sha2-256-etm@openssh.com', u'hmac-
sha2-512-etm@openssh.com', u'hmac-sha1-etm@openssh.com',
u'umac-64@openssh.com', u'umac-128@openssh.com', u'hmac-sha2-256', u'hmac-
sha2-512', u'hmac-sha1'] client compress:[u'none', u'zlib@openssh.com']
server compress:[u'none', u'zlib@openssh.com'] client lang:[u''] server
lang:[u''] kex follows?False
ssh: Kex agreed: diffie-hellman-group1-sha1
ssh: Cipher agreed: aes128-ctr
ssh: MAC agreed: hmac-sha2-256
ssh: Compression agreed: none
ssh: kex engine KexGroup1 specified hash_algo <built-in function
openssl_sha1>
ssh: Switch to new keys ...
The authenticity of host 'myserver.mydomain.mytld' can't be established.
SSH-RSA key fingerprint is zzzz.
Are you sure you want to continue connecting (yes/no)?

I am using duplicity 0.7.15 on CentOS Linux 7, with Python 2.7.5
```

Story points: 6.0