lxc-start: if we pass in a config file, then don't use any loaded config

To do this, add a c->clear_config() helper to the api.

(this fixes the bug https://bugs.launchpad.net/bugs/1251352)

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