OpenSSH 7.0 compatibility for kvm gitian build

[laanwj]

OpenSSH 7.0 no longer has dsa enabled by default:

debug1: Skipping ssh-dss key ./var/id_dsa for not in PubkeyAcceptedKeyTypes

This works around that. Another possibility would be to switch the key types but the other 'fast' key types such as ssh-ed25519 aren't consistently supported in the targeted Ubuntu versions, and that would require regenerating base images, so I think this is the best workaround.

@luke-jr tested it

[maflcko]

@laanwj Haven't yet looked closely but is this relevant after #107?

[laanwj]

I don't think so, although in your case the base images have to be regenerated, not a big deal I guess...