In this change the new OpenStack-API-Version headers is allowed,
but not required, for requesting a microversion.
Both headers are accepted in the request and both headers are sent in
the response (both the header and its value, and the addition to the Vary
header).
Many tests which explicitly use a microversion header have been
updated to use both. This change is not 100% as most of the tests
are testing the handling of the value of the header, not which
header is involved.
Partially-Implements: blueprint modern-microversions
Change-Id: I68da13b5ba0c2f3357523e765a5b9db81899daf1