- New dict setting VMS_USER_QUOTA to allow setting per user quota
  (overrides MAX_VMS_PER_USER setting)
- Private networks quota mechanism (based on MAX_NETWORKS_PER_USER and
  NETWORKS_USER_QUOTA settings)
- Different response message for GANETI_MAX_LINK_NUMBER overlimit
  errors (To differentiate from user quota overlimit error). Backend
  not raises NetworkLink.NotAvailable exception when no available link
  can be created.
- Additional unittests for vms/networks quotas