LP#1879790: Give carousels their own context org

LP#1879790: Give carousels their own context org

Before this commit, the context org unit for carousels was the same as
the physical_loc environment variable, or the client-passed physcial
location, or the search location, whichever was set in that order.  This
commit provides the ability to specify a carousel_loc environment
variable in the apache vhost configuration, as an override to
physical_loc.  This, in turn, allows hostname-segregated branches of the
org hierarchy in a resource sharing group to specify their own carousels
even when they want the top of the group to be the default search
location.

Signed-off-by: Mike Rylander <mrylander@gmail.com>