Add shared parameter for metering labels

With this patch metering labels can
be set as shared so that the rules
associated with this label will be
applied to all routers for all tenants.

Partially implements
blueprint l3-metering-mgnt-ext

Change-Id: I23f9fb9cf17c7a20069616bb1c4ae19a5dc55d68
Merge "Add shared parameter for metering labels"