app: Add debug mode

Can be used to debug an application using pdb, e.g.:
$ PYTHONPATH=src/ python -m pdb src/ovirt/node/setup/__main__.py \
                                --dry \
                                --defaults /tmp/cfg_dummy \
                                --debug

Without the switch every exception will be catched and thus not passed
to pdb.

Change-Id: Ib0bf1b3d6bb360b3dfda9bb7c604153e0f13bebc
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>