Allow cassandra binary path to be configured through env variable

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 SWH_CASSANDRA_BIN.

This also:

• fixes an issue on log path inexistence which raises.
• renames the other env variable LOG_CASSANDRA to SWH_CASSANDRA_LOG (for consistency)