Fix logic for handling SNAT rules

Bug 1192610

Fixes and simplifies the logic for managing SNAT rules, based
on the assumption that a chain contains SNAT rules for a single
router.
It also fixes another small glitch with SNAT rules not being
removed when a gateway port is destroyed (the glitch did not
affect operations)

Change-Id: Ia95e375459a1f32e93bbe912a268a8ed13859c69
Merge "Fix logic for handling SNAT rules"