NSX: add nsx switch lookup to dhcp and metadata operations

The operations in questions were still using the neutron network
uuid as the nsx switch uuid, but this is no longer valid.

Close-bug: #1312439

Change-Id: I0f4bf02bb176c5c3914c7dd7e8249121af1b5a79
(cherry picked from commit 0f2f768578900a7ca74091ab400bd9facce9eaf9)