Currently the vm to use to run the slave is searched in several directories and
if one is found it is used.

As this is not really predictable when running a slave which is not only a slave
there should be a possibility to definitley specify a vm to use and only search
for one if none is specified.

Especially as we are not installing the vms to the default locations as they are
used by our operators for other things. We do want a slave being completely in a
sandbox having it's own vms, maven settings etc. Currently this is possible
except the vm to run the slave