When Avocado runs tests on remote machines it always accepts the
host SSH key fingerprint. This could allow credentials to be stolen
if the remote host name or address is spoofed.

This patch adds to Avocado the configuration option to enable/disable
the reject_unknown_hosts option, so user can choose between safe or
flexible.

Notice there is an issue in Paramiko where it does not recognizes
ecdsa keys in known_hosts file. So for reject_unknown_hosts to work,
we currently have to accept the remote host key fingerprint in RSA format.
To do so, the SSH command shouled be like below:

  $ ssh -oHostKeyAlgorithms='ssh-rsa' <host>

Reference: https://trello.com/c/oqyAvZBqSigned-off-by: NAmador Pahim <apahim@redhat.com>