Add wsgi_chunked_request option

This option is necessary[1] when running OpenStack's keystone on Apache.
This functionality has been merged[2] into the upstream puppetlabs-apache.

[1] https://review.openstack.org/#/c/34835/
[2] https://github.com/puppetlabs/puppetlabs-apache/pull/890

Change-Id: Icaf550e6570890535c7f43e6c77889826bfa90cb