python: Various fixes to the python scripts

This fixes a few issues uncovered by the recent C module fix.

In lxc-start-ephemeral, the hwaddr code wasn't actually working.
Replace by code that properly iterates through the network interfaces
and sets a new MAC address for each entry.

In the python overlay, catch the newly emitted KeyError when in
set_config_item (or setting any previously unset variable would fail).

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>