This makes the servers API look up the CellMapping for the instance
in question and target the context to the appropriate cell for the
subsequent call into compute/api.
Note that just as with get() and get_all(), we need to avoid going
direct to the cell database and thus short-circuiting the replication
in the case of cellsv1.
Related to blueprint cells-aware-api
Change-Id: If63a4fa7349890a6e7ac0fa3fe12917dc53334d0