cassandra: add option to configure logging

As cassandra start script hardcodes the location of logback
configuration to `CASSANDRA_CONF_DIR/logback.xml` there is no way to
pass an alternate file via `$JVM_OPTS` for example.

Also, without logback configuration DEBUG level is used which is not
necessary for standard usage.

With this commit a default logback configuration is set with log level
INFO.

Configuration borrowed from:
https://docs.datastax.com/en/cassandra/3.0/cassandra/configuration/configLoggingLevels.html