split setup and execution features in sandbox scripts

The sandbox scripts, such as start, stop, use, have all two parts. One deals with setting up the environment variables, and one runs the applications.
It is desirable to split these two features, to have a sandbox-env.sh, called by all the scripts, which in turn will only contain the commands to run.
The rationale is to allow easy scripting by advanced users who need to run the sandbox in ways that are not easily achievable with the current architecture.