Adds API microversion response headers

Adds the following headers to REST API headers for API
microversions:

X-OpenStack-Compute-API-Version: Version
Vary: X-OpenStack-Compute-API-Version

Partially implements blueprint api-microversions

Change-Id: I5748d7b81b355bf8aad6bb8703d1a29a97798452
Merge "Adds API microversion response headers"