Add API versioning similar to what Ironic does

Every request may have X-OpenStack-Ironic-Inspector-API-Version header with version in form of "X.Y". Error 406 will be returned, if unknown (or unsupported) version is supplied. Minimal known (1.0) version will be assumed, if header is missing.

In every response X-OpenStack-Ironic-Inspector-API-Minimum-Version and X-OpenStack-Ironic-Inspector-API-Maximum-Version will be returned, again in form of "X.Y". Minimum version should be "1.0" for now and foreseeable future.

Both client functions will accept "api_version" argument, either as 1 number (major version, use the default) or as a tuple (X,Y), or as a string "X.Y". Client will have DEFAULT_API_VERSION attribute set to (1,0).

No effort will be undertaken to prevent a user from NOT declaring a needed API version (as opposed to what Ironic and Nova are doing).

Server change:
* Return API versions in *every* response
* Check that provided API version is supported

Client change:
* Actually pass headers to the server
* Implement server_api_version() function which does GET /
* If api_version is provided, call server_api_version() to check it _before_ issuing any request (TODO: caching)

Reference:
* https://review.openstack.org/#/c/187112/ - API-WG spec API versioning
* https://review.openstack.org/196320 - Ironic spec on finishing API versioning