lnst-ctl: do not use user pool directory if global one exists

If there's a pool directory set in the system wide configuration file
it is overwritten on the first run of lnst-ctl. The patch changes this
behavior so that the user's pool directory is used only if the system
wide config does not set the pool directory.

Signed-off-by: Jan Tluka <jtluka@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>