A lot of clouds using the router service plugin don't configure for DVR,
but the service plugin still loads the extension, and exposes it via
API. Which will break if api consumers (admins with default policy.json)
attempt to create new style routers based on the information passed
through /extensions/ api.
This change introduces a new config option that allows to avoid loading
the extension. For complatibility sake, it requires an opt-in from ops
side to disable it, otherwise the extension is still loaded as before.
This is helpful for automation matters. It may also be useful when
preparing tempest.conf api_extensions=, when you could actually pass the
result of /extensions/ request into tempest and expect the test suite to
pass without yanking dvr off the list for non-dvr setups.
We could go further and try to check if the controller is configured
properly. That is complicated by the fact that f.e. such validation may
require talking to ml2 drivers, or even agents, which is not feasible
during api startup.
Change-Id: I84be9be93862fe71a2d5b5322d7ebd476c784163
Related-Bug: #1450067
da8d5b4770