Commit 67702c21 regressed the case where lxc-create use a config
file with 'xx:xx' in lxc.network.hwaddr, so that the 'xx' were
preserved in the container's configuration file.  Expand those
in the unexpanded_config file whenever we are reading a
config file which is not coming from a 'lxc.include'.

The config file will have \n-terminated lines, so update
rand_complete_hwaddr to also stop on \n.

Add a test case to make sure xx gets expanded at lxc-create.
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>