pass lxc_conf to the lxc_start function instead of the rcfile

pass lxc_conf to the lxc_start function instead of the rcfile

The rcfile is parsed in the lxc_start function. This is not the place
to do that. Let's the caller to do that.

In the meantime, we have the lxc_conf structure filled right before
calling the lxc_start function so we can do some sanity check on the
configuration to not break the system when we launch the container.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>