During router create, if we are also updating external gateway, we were updating the logical_router object without reading it. This could clobber the id_perms on the server. We should read before updating the object. Ideally, we should protect against this in api server.

Change-Id: I529b7f9e0b2ab3b635809af8678addc34467422c
Closes-Bug: 1373257
(cherry picked from commit f1342cfddd940e85c08d763d935db12f7aa66d91)
Merge "During router create, if we are also updating external gateway, we were updating the logical_router object without reading it. This could clobber the id_perms on the server. We should read before updating the object. Ideally, we should protect against this in api server."