#16 Implement Certificate Fingerprinting

We should have the VEN print their own certificate's fingerprint on startup (if a certificate is provided), which can be shared with the VTN to identify the VEN.

From the OpenADR documentation:

import ssl, hashlib
bin_cert = ssl.PEM_cert_to_DER_cert( open('client_cert.pem').read() )
sha_hash = hashlib.sha256(bin_cert).digest()
print ':'.join( '%02X' % ord(c) for c in sha_hash[-10:])

This also means that the opposing party should be able to use the certificate fingerprint (instead of the certificate itself) to authenticate the incoming messages.