fix "if" condition for mx4j tool in cassandra-env.sh

There is this in cassandra-env.sh

if [[ "$MX4J_ADDRESS" == \-Dmx4jaddress* ]]; then

(similar for port)

This is wrong for /bin/sh shell (our shebang in bin/cassandra) and this does not work, probably in bash only, because /bin/sh does not understand what "[[" is nor it understand what "==" is.

The reason this was never detected so far is that the logic will never come there when MX4J_ADDRESS and / or MX4J_PORT is commented out couple lines above.