Improve BlockDeviceMapping object cells awareness

This patch makes BlockDeviceMapping object methods that are creating and
updating the database more aware of the cells. There are 2 changes that
are good to distinguish between:

 - create() will _never_ work in the API cell and will raise an
   ObjectActionError.
 - save() and destroy() are now aware in which cell they are being
   called and do not attempt to propagate it upwards

This is currently a no-op change as we don not currently use objects in
any of the code that will run in the top cell. This is in fact
preparation for transiting the API code to using BlockDeviceMapping
objects.

Change-Id: Ic5806c844f2deab734011d68f5da316903000f67
Merge "Improve BlockDeviceMapping object cells awareness"