This simplifies handling of only part-time active clients

This is useful for experiments where not all clients are active right from the start

When passing '--check' as first argument, then an experiment
will run a basic configuration check.

Previously only variant options were checked to actually override an
entry from refit-defaults. This now applies to all override options.

The implementation has moved to benchmark.py as JavaTestRun.
Wrapping the command with ssh is now a separate function and
gen_remote_java_cmd now also accepts an optional command prefix.