set gnocchi stable/2.1 to use trusty
set gnocchi stable/2.1 to use trusty

gnocchi stable/2.1 is mapped to stable/mitaka. therefore,
we need it to run against trusty or else devstack won't run.

Change-Id: I2f10e03c798943f6035bdd795cf3ea9f89e4d582

openstack-infra/project-config,openstack-infra/project-config