When a container was created, its configuration is used.
When a container was not created, the configuration specified in
the command line is used, if not configuration file is used,
default values are used.

That allows to create 'volatile' container, like tmp files.
It is useful for example to spawn different container with the
same generic configuration file. That let the user to have its own
repository of configuration files.

And, more important, that fix temporary created container with
lxc-execute to be not deleted when the host crash or the command
is killed.
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>