authentication broken

With enabled authentication the example omf/omf_ec/example/test_exp/test02.rb fails due to the fact that when a FRCP create message arrives, a certificate for the child resource is created and signed by the parent certificate, but the parent certificate is not on the certificate store, thus the child certificate cannot be verified. Introducing a new store (@intermediate_store) solves this problem. 
Also this is fixing a typo and modifying bin/omf_cert.rb to be able to take a --root argument that signs the new certificate.

This fixes this issue: https://github.com/mytestbed/omf/issues/6.