Move _generate_paste_config to tests.core

Seeing as how tests for oauth1 and endpoint filtering each have
their own private functions for creating a new paste-ini file,
I think it's best to move these to tests.core.
Future extensions will benefit from this too.

fixes bug: #1218720

Change-Id: Ic31b78dda0a66ddc4c4214bca4ba3b683d554886
Merge "Move _generate_paste_config to tests.core"