python3: Don't require a template name

The template name isn't required, if it's not passed, then create will
simply be asked to create a container without a rootfs.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>
python3: Handle invalid global config keys

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