Call unbind_snat_servicenode from schedule router

Refactor to move the call to plugin.unbind_snat_servicenode from
schedule_snat_router to _schedule_router.  This is a move to pave the
way for removing hints from schedule router.

Partial-bug: #1353266
Partial-bug: #1356639
Co-Authored-By: Swaminathan Vasudevan <swaminathan.vasudevan@hp.com>

Change-Id: I062d8cc3cb870bbaa033d5b107a7dd868dfafa19
Merge "Call unbind_snat_servicenode from schedule router"