Add flag to stop swob from always using absolute location.

This is a backport of commit 89deaf5f3cb062cc2c2d7d7d365e84ec2e41a511

This is needed for SOS (along with patch
https://github.com/dpgoetz/sos/pull/37)
to work with swift 1.12 . By spec you should always use the absolute
location but this causes a problem with staticweb over a cdn using a
cname. Basically you want to be able to forward the browser to a
relative location instead of whatever full url the proxy server
thinks you are using.

Change-Id: I28dcbd779d2aa4a6e636c956b058670b8e4b2fd7