cassandra.yaml - actually separate server_encryption_options and client_encryption_options

Currently they have different names but use the same option names, which doesn't make sense for two reasons:

• internode_encryption: none/all doesn't make sense for client encryption. Should be 'enabled: true/false'
• truststore and truststore_password are not used by Cassandra for client encryption and shouldn't be present in client_encryption_options.

Moved type-dependent options to either ServerEncryptionOptions or ClientEncryptionOptions. Left truststore and truststore_password in EncryptionOptions for use by Stress and CLI (but removed them from cassandra.yaml).