When require-client-auth (SSL) is enabled on server, astyanax client does not work

- When Cassandra/DSE server nodes have require-client-auth specified to true, the clients need to send client cert for validation with the server nodes.  Didn't currently find a way to do with astyanax client.  Adding minor changes to accomplish that.

- When astyanax client is configured with sslcontext, now there will be a way to add client keystore using the withKeyStore method of SSLConnectionContext class