Add support for microversions API special version latest

Adds support for specifying 'latest' as the version of the API in
the X-OpenStack-Compute-API-Version header. This will always
behave as if a request was made for the maximum version
supported.

Partially implements blueprint api-microversions

Change-Id: Ib40b20a01991e6f356b9376f797405a4c50b7e99
Merge "Add support for microversions API special version latest"