I missed this the changes from I4ee0110a6c2b9466d81e37e5df27f5f81a6eceb5 when
rebasing I9f40a2a3561fcb1d1fec9d9c3c1f9cabaf02650c.
Add the missing key to the roles and add some validation to catch this.
Change-Id: I0f38df69777340ebbdc5c419f121524dbc74cad7
Related-bug: #1831617
Implements: blueprint nova-backend-per-role