Allow on-demand scanning using openssl

[daveschaefer]

We should add a switch to allow notaries to use openssl for on-demand scans in addition to using the built-in python scanner. This should be as simple as importing a different attempt_observation_for_service(); from ssl_scan_openssl.py rather than ssl_scan_sock.py.

People setting up their scanning process can of course already run it with the desired python module. This switch would change the behaviour of notary_http.py for on-demand scans.