This commit adds an API enabling the POST operation to create scopes in
an ad hoc fashion. This is useful for operators to register scopes
before they are created as resources in the collected backend so their
processing can be discarded right away, for example for trial
projects/accounts.
Otherwise, we need to wait for them to create resources, then for
example Ceilometer has to monitor these resources, persist measures in
Gnocchi, then CloudKitty has to discover the scopes and finally we can
disable their processing.
Change-Id: I3e947d36c9d5d5da07115d35dde578ae300cbe5c
3a2623484e