Add option for defining certificate data in external pem files

The current strategy of embedding certificate contents in the Repository settings blosts the settings considerable with unreadable data, and introduces a manual step of inserting the binary certificate payload into the RepositorySettings.xml file.

If we instead added a alternative element <CertificateLocation> to <CertificateData>, it would be possible to just define a pointer to the actual pem file.

The pem file would then need to be distributed together with the RepositorySettings.xml.

A added benifit of this would be that normal certificate (openssl) tools can be used directly on the pem certificates.