lxc-checkconfig: Allow to override grep and config

lxc-checkconfig: Allow to override grep and config

It might be handy for the user to specify a different kernel config file
to check, perhaps the one of a kernel he's about to build. To allow that
we only set the CONFIG variable if it's not present in the environment
before. Also, if CONFIG is not found and we resort to a different file,
we say it explicitely, to avoid typos on the user's part resulting in
silently checking a different config than the one the user wanted.

Signed-off-by: Guido Trotter <ultrotter@quaqua.net>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>