(#8193) Support new keytypes for sshkey
Recent versions of openssh (at least openssh 5.8) support the following
keytypes in in the authorized_keys file: ecdsa-sha2-nistp256,
ecdsa-sha2-nistp384, ecdsa-sha2-nistp521, ssh-dss or ssh-rsa.
Add support for ecdsa-sha2-nistp256, ecdsa-sha2-nistp384 and
ecdsa-sha2-nistp521.