packaging: setup: Do not fail with missing /etc/sysconfig/nfs

Make engine-setup do not fail if /etc/sysconfig/nfs does not exist,
instead behave as if it was empty. engine-cleanup leaves it behind
empty.

Bug-Url: https://bugzilla.redhat.com/1021867
Change-Id: I62b22a12b9abd408657696caa8d34243ae521cbd
Signed-off-by: Yedidyah Bar David <didi@redhat.com>