L3 Metering label as shared

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.

Also changed all attributes on metering labels
to not allow updates ('allow_put': False),
since there are no update methods.

Partially implements
blueprint l3-metering-mgnt-ext

DocImpact

Change-Id: Ice405585fc50786d52eecc35c01605ac0e9550ac
Merge "L3 Metering label as shared"