edit-node: Fix iso filename creation

Previously a naïve approach was choosen to build the filename of the
edited iso. But this approach had problems when RPMs were passed,
instead of package names.
Now a couple of checks an a structured appraoch is used to build a sane
and nice final isoname.

Change-Id: I2430c392cb48fbe8ba7c345be485cc8db14fa598
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>