Move _convert_to_nsx_transport_zones into nsx_utils

This routine does not need to be in the plugin class. This patch
transforms it into an utility method and adds unit tests to
increase coverage.

Change-Id: Ieacc36e729c4db90efd33944f7cf05dcb30e1de9
Closes-Bug: 1340210
Merge "Move _convert_to_nsx_transport_zones into nsx_utils"