rgw_rest: Make fallback uri configurable.

Some HTTP servers, notabily lighttp, do not set SCRIPT_URI, make the fallback
string configurable.

Signed-off-by: caleb miles <caleb.miles@inktank.com>
Reviewed-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit b3a2e7e955547a863d29566aab62bcc480e27a65)

Conflicts:
	src/rgw/rgw_rest.cc