Make get_oauth_params conditional for specific oauthlib versions

With recent change to oauthlib 0.6.2, the get_oauth_params method
now takes an extra argument. This patch will change the behaviour
of test method _validate_oauth_headers, which will call
get_oauth_params with or without the extra argument, depending
on the version of oauthlib detected.

Change-Id: Id82c28993fa735b8c5b1a4d64ed2f55cc2eddbba
Closes-Bug: #1327430