packaging: fix default filemod in copyFile

copyFile used to call os.chmod(file, filemod) with filemod defaulting
to '-1'. Older linux kernels kept the mode unchanged in such a case, but
newer ones don't. More details in the "Bug-Url" link below.

This change defaults filemod to None and only calls os.chown if filemod
is set.

Bug-Url: https://bugzilla.redhat.com/1011616
Change-Id: I9e436d72e8ffb8d48e05201727d5b46797881556
Signed-off-by: Yedidyah Bar David <didi@redhat.com>