When trying to add interface from a shared network by other tenant to
router, internal server will be raised.
Should use router tenant id instead of subnet tenant id to create port
when adding interface to router. And from the user's perspective the
port created should belong to the user who created it.
Change-Id: Ib124bbc85b9289940eb3de75a222196479e48e6b
Closes-bug: #1673696