[sdk-vm] install /etc/ssh/authorized_keys in post to avoid issues when used as a ro mountpoint

Signed-off-by: David Greaves <david.greaves@jollamobile.com>
Use a specific ssh config to avoid problems with .ssh/known_hosts

Signed-off-by: David Greaves <david.greaves@jollamobile.com>
Release mer-0.55-1