Add flag to stop swob from always using absolute location.

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: I3fa1d415bf9b566be069458b838f7e65db0c4f39