lxc-start-ephemeral: Fix console() and add storage option

lxc-start-ephemeral: Fix console() and add storage option

The introduction of the new console() python API broke
lxc-start-ephemeral's console(tty=1) call, I now changed that to
console() which does the right thing with both API versions.

This also adds a new storage-type option, letting the user choose to use
a standard directory instead of tmpfs for the container (but still have
it ephemeral).

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>