edit-node: Fix filename generation for non-nvra

Previously the filename suggestion for the output iso filename failed if
the input filename didn't follow the nvra scheme.
Now a simple suggestion is used as a fallback.

rhbz#965432

Change-Id: I3900ad6b112aa8224e13d9385b5b966f007b3857
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>