Use 'generate_request_id' func from common code

There are two reasons to use it:
1) Removal of duplication
2) In Python 3, webob will return bytes, so we need to fix
'generate_request_id' func so that it also returns bytes.
The behaviour is unchanged in Python 2.

Partially implements use-common-code

Change-Id: I4627e290c0b5c71fcfdbd1f88c5f592385bdd4f5
Merge "Use 'generate_request_id' func from common code"