Tweak version nvp/nsx version validation logic for router operations

This patch improves how the nsx/nvp controller version is validated
prior to some router operations. This is done by greatly simplifying
the boolean condition.

Missing unit tests are also added for increased coverage.

Closes-bug: 1274361

Change-Id: I1b21444fd86b6e6a0c72bb5a43f1707934dd70e4
(cherry picked from commit f055df48bf7a605463b28b66f5197f1275fb6948)
Merge "Tweak version nvp/nsx version validation logic for router operations" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable