This is a preliminary change for the counting quotas series to make
any legacy quota "reservations" arguments default to None so that in
the next change, we can simply not pass any reservations instead of
passing empty reservations. Reservations are being deprecated as part
of the counting quotas series.
Part of blueprint cells-count-resources-to-check-quota-in-api
Change-Id: I86e56c1abd979ac9968a3ce7daad07530080bde4