The requirements/upper-constraints file was modified to allow WebOb
1.8.1 by commit 88bafa11deb9bd7595983d97ffabca338f073ba3. This
patch simplifies some Glance code that could handle both WebOb
1.7.4 and 1.8.1 so that it now only handles the latter, and updates
the glance/requirements.txt and glance/lower-constraints.txt to
reflect that we support WebOb 1.8.1+ only.
Change-Id: I03e03013927cc5434aa0d97657d5e7efd8223ee5
Closes-bug: #1770410
Brian Rosmaita