devstack: fix plugin with rename of get_or_add_user_role
devstack: fix plugin with rename of get_or_add_user_role

It has been renamed in devstack in commit 9b215db569dcee2e9cd52c3336ba14b73ad05ab0.

Change-Id: I5f3ce41e63cc4775d50973bd3e95610b53dd9c8b

gnocchixyz/gnocchi,sileht/gnocchi,idegtiarov/gnocchi-rep,leandroreox/gnocchi,idegtiarov/gnocchi-rep,leandroreox/gnocchi,sileht/gnocchi,gnocchixyz/gnocchi,idegtiarov/gnocchi-rep