WebOb 1.7 changed [0] how request bodies are determined to be
readable. Prior to version 1.7, the following is how WebOb
determined if a request body is readable:
  #1 Request method is one of POST, PUT or PATCH
  #2 ``content_length`` length is set
  #3 Special flag ``webob.is_body_readable`` is set

The special flag ``webob.is_body_readable`` was used to signal
WebOb to consider a request body readable despite the content length
not being set. #1 above is how ``chunked`` Transfer Encoding was
supported implicitly in WebOb < 1.7.

Now with WebOb 1.7, a request body is considered readable only if
``content_length`` is set and it's non-zero [1]. So, we are only left
with #2 and #3 now. This drops implicit support for ``chunked``
Transfer Encoding Glance relied on. Hence, to emulate #1, Glance must
set the the special flag upon checking the HTTP methods that may have
bodies. This is precisely what this patch attemps to do.

[0] Pylons/webob#283
[1] https://github.com/Pylons/webob/pull/283/files#diff-706d71e82f473a3b61d95c2c0d833b60R894

Closes-bug: #1657459
Closes-bug: #1657452
Co-Authored-By: Hemanth Makkapati <hemanth.makkapati@rackspace.com>
Change-Id: I19f15165a3d664d5f3a361f29ad7000ba2465a85