StoreCertificateInOverlay

void MessageTransport.StoreCertificateInOverlay()
encapsulates the storage of a nodes certificate as
CERTIFICATE_BY_NODE and CERTIFICATE_BY_USER
Fetch CertificateStore Test

In  ImageStoreUsage.AppProcedure() we test to fetch a stored certificate
from the overlay.
There is a comment that tries to describe the intention for that.

Some small changes in CertificateStore usage