fix object_exists reference
fix object_exists reference

we use a global context now.

Change-Id: I2ea2aba4aa7a0364f48ec59bb7b1a38e22d3028e

gnocchixyz/gnocchi,gnocchixyz/gnocchi,leandroreox/gnocchi,sileht/gnocchi,leandroreox/gnocchi,sileht/gnocchi