packaging: engine-setup avoid traceback when loading iso

In engine-setup, _loadFilesToIsoDomain now try to copy the iso files
provided by possibly installed rpm into the configured ISO domain
only if they exists avoiding useless traceback in the log.

Change-Id: Ic995c3bee295b964d2f6a729e4d4fb750eb6b4b5
Bug-Url: https://bugzilla.redhat.com/904763
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>