functional_tests: VM with debian8-base image isn't reachable with SSH

[gbenhaim]

vm_name = 'vm-debian8-base'

    def test_vms_ssh(vms, vm_name):
        vm = vms[vm_name]
>       assert vm.ssh_reachable(tries=200)
E       assert False
E        +  where False = <bound method DefaultVM.ssh_reachable of <lago.vm.DefaultVM object at 0x7f83b39f3090>>(tries=200)
E        +    where <bound method DefaultVM.ssh_reachable of <lago.vm.DefaultVM object at 0x7f83b39f3090>> = <lago.vm.DefaultVM object at 0x7f83b39f3090>.ssh_reachable

The boot logs from the VMs can't be examnied because log collection fails on #700