config: Modify/use server_encryption_options
config: Modify/use server_encryption_options

* Mark option used
* Make sub-options adapted to seastar-tls useable values (i.e. x509)

Syntax is now:

server_encryption_options:
	internode_encryption: <none, all, dc, rack>
	certificate: <path-to-PEM-x509-cert> (default conf/scylla.crt)
	keyfile: <path-to-PEM-x509-key> (default conf/scylla.key)
	truststore: <path-to-PEM-trust-store-file> (default empty,
                                                    use system trust)

raphaelsc/scylla,scylladb/scylla,kjniemi/scylla,avikivity/scylla,avikivity/scylla,kjniemi/scylla,kjniemi/scylla,raphaelsc/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,duarten/scylla