[Java container] $FABRIC8_JVM_ARGS overrides etc/jvm.config

We should store the values passed to the Fabric when creating Java Container (JVM options, main class, application arguments). Otherwise we won't be able to execute managed processes via `bin/launcher start`, because these settings won't be remembered by the process itself.

We can't force users to execute processes only via Fabric - that would be a devops hell. If Fabric is down for some reasons, folks still should be able to manage their processes manually. That would be production-saver.

I changed launcher slightly, so it tries to read `$FABRIC8_JVM_ARGS` environmental variable and fallback to the `etc/jvm.config` if the former is empty. This way Fabric can overrides value cached in the process directory.

BTW We should consider using process IO API (#1354) to update files like `etc/jvm.config` when the values in the `io.fabric8.container.java` PID changed.