python WebOb dependency made unpinned.

keystone declares python-webob dependency as ==,
which is not a good way of defining dependency.
So to support latest dependencies, WebOb is unpinned in
requirements.txt

Fixes: bug #1180033
Change-Id: I73042cf88baf378c8564abb2394963866082977e
Merge "python WebOb dependency made unpinned."