packaging: engine-backup - support old md5sum

Removed '--strict' md5sum option, which isn't supported on earlier
versions of md5sum.

Change-Id: I723bc81e84be05aa2fb3ca2ceb8210a2bfe7adc9
Signed-off-by: Ofer Schreiber <oschreib@redhat.com>
Bug-URL: https://bugzilla.redhat.com/1003869