Avoid an extra database query in schedule_snat_router

bind_snat_servicenode already knows the chosen_agent.  By waiting
until after it is called, we can use it to avoid another db query.

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

Change-Id: Iba5668b2717e188d5289ede982bfc1b46f4eeb5b