main: Fix implicitly disabled client encryption options
main: Fix implicitly disabled client encryption options

The start_native_transport() function in storage_service expects the
'enabled' option to be defined. If the option is not defined, it means
that encryption is implicitly disabled.

Fixes #718.

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