The current hard-coded value won't work for other distributions not relying on
standard conventions (e.g. nixos...).
This keeps the original behavior and only allow to diverge based on the
environment variable BIN_CASSANDRA 
- fixes an issue on log path inexistence which raises.
- renames the other env variable LOG_CASSANDRA to SWH_CASSANDRA_LOG (for consistency)
Currently trying to run this with my nixos box and this minimum prevents it.