Add plural names for quota resources

Add plural names for quota resources

Introduce a 'plural_name' attribute in the BaseResource
class in the neutron.quota.resource module. This will
enable to specify the plural name of a resource when it's
entered in the quota registry.

This will come handy for managing reservations and also
removes the need for passing the collection name when
counting a resource.

Also, the name of the 'resources' parameter for the
_count_resource routine has ben changed to collection_name,
as the previous one was misleading, since it led to believe
it should be used for a collection, whereas it is simply
the name of a collection.

Change-Id: I0b61ba1856e9552cc14574be28fec6c77fb8783c
Related-Blueprint: better-quotas