enable_proxy_headers_parsing should not only be configurabe if
!$nova_metadata_wsgi_enabled , Therefore it needs to be moved
outside the condition.
But at the moment enable_proxy_headers_parsing can also be set via
::nova::metadata which conficts.
This change deprecates oslo::middleware related parameters
enable_proxy_headers_parsing and and max_request_body_size in
::nova::metadata. In future ::nova::api should be used instead.
Change-Id: I0f138bf278931ac21d228251cc1da44638da915c