Require a CA Cert in a SecurityCreds object and via SSL

The initial version of security would allow a CA Certificate, but would not require one.  Now when using a `SecurityCreds` object you must supply a CA Cert.  This CA Cert option can now also be a list of cert files.

At the library level, OpenSSL will now verify the peer certificate (from the server) with the list of CA certs cached on the client.
But of course `py` is the default domain, so we can just use the simpler notation you suggested.