Fix #141: NPE in com.sixsq.slipstream.persistence.Run.validateQuota

We moved everything which is quota related to dedicated classes
``com.sixsq.slipstream.persistence.QuotaParameter`` and
``com.sixsq.slipstream.run.Quota``. Also, we now also check the
connector quota value when user one is not set before resorting to the
system value.