Failed to allocate 17.* LXD systems

[Saviq]

Given a backend:

    lxd:
        systems:
            - ubuntu-16.04
            - ubuntu-17.04
            - ubuntu-17.10

Spread fails to allocate the 17.* systems:

2017-10-24 15:18:39 Discarding lxd:ubuntu-17.04 (spread-12-ubuntu-17-04), cannot connect: cannot connect to lxd:ubuntu-17.04 (spread-12-ubuntu-17-04): ssh: handshake failed: ssh: unable to authenticate, attempted methods [none password], no supported methods remain
2017-10-24 15:18:43 Discarding lxd:ubuntu-17.10 (spread-14-ubuntu-17-10), cannot connect: cannot connect to lxd:ubuntu-17.10 (spread-14-ubuntu-17-10): ssh: handshake failed: ssh: unable to authenticate, attempted methods [none password], no supported methods remain

[kyrofa]

This happens for 18.04 as well.