Fix typo and use default cert

This script has a typo that prevents successful execution.  In
addition, the usage section describes defaulting to a default
certificate, but that logic does not exist in the script.  The script
is updated to:

* Change ENIGNE to ENGINE
* Handle the default cert
* Exit if default cert does not exist and no cert specified
* Exit if specified cert does not exist

Change-Id: I4b04c74ccc62664ead27339566e540bdd717eb71
Bug-Url: https://bugzilla.redhat.com/1082506
Signed-off-by: James W. Mills <jamills@redhat.com>
Signed-off-by: Eli Mesika <emesika@redhat.com>