Clearer definition (+ bug fix) for min/max quotas

Now min quota considers the number of running VMs owned by HTCondor as
the current total number of running VMs, whereas max quota considers
the number of *all* running VMs, also the ones *not* owned by HTCondor,
to avoid cloud overflows due to a request rate too high or the presence
of misconfigured VMs that never join the HTCondor cluster.