Pass C* backend option via Env-variable

Currently to start Hawkular with something else than the embedded C* database, one has to pass some -D.. switches on the command line.

I think it would be good to allow for them to be passed in via Environment variable.

• One does not forget to pass the options on repeated starts
• In Docker images, the start path / command is encoded in the Dockerfile. Modifying it to pass -D... is not possible on when executing 'docker run'.

Now the "most straightforward way" is probably to modify JAVA_OPTS and append them there. I think this is not optimal, as one has to repeat all the options that are normally set inside standalone.conf into this 'enhanced' version.

I propose to have extra env-variables HAWKULAR_CASSANDRA_BACKEND = ... and _BACKEND_NODES

That Hawkular will use. If a -D switch is present, it should have priority. If nothing is given, some default as today is used.