Update common openstack code from oslo-incubator

* Update to oslo-incubator commit b17b268a269c4989d76267db5c2d49d4c20bd51d.
* Update usage of 'deferred_version_string' to 'cached_version_string'
* Add context values 'user' and 'tenant' for context-logging
* Remove prefixes that resolve to '%prog' in --version CLI output

Change-Id: I24d9a24ad1a6e9379008ea719c9cbd22899111f9
Set content-length despite webob

Webob has a tendency to wipe out the content-length we set on responses.
This causes eventlet to use chunked transfer encoding. By changing the
order of operations, however, we can trigger eventlet to use the simpler
and more informative Content-Length header.

Change-Id: I0ed27c9e668a538bc0fc29cb6e9127dfb7fd8c78
Merge "Update common openstack code from oslo-incubator"