Unuse the mapping_uuid field in cascading

The added 'mapping_uuid' field in Instance table in DB of cascading
store the mapping relationship between cascading and cascaded instances.

We replace the above method to putting the mapping into a dict in the
memory for improving performance. Another modification is sync flavor
when reboot.

Change-Id: I2e6cf2a3eb9649509e5be181631c2030104a3f9b