When an unprivileged user want to access to Gnocchi resources
created by Ceilometer, that doesn't work because the filter scope
the Gnocchi query to resource owner to the user. To fix we introduce
a new configuration option "gnocchi_external_project_owner" set by
default to "service". The new filter now allow two kind of Gnocchi
resources:
* owned by the user project
* owned by "gnocchi_external_project_owner" and the orignal project_id
of the resource is the user project.
Change-Id: I0c86736a902a21520da18550aea0a7d1549bb24e
b0bdd43209