MB-18024: fix head frag inclusion code under multiple docroots
MB-18024: fix head frag inclusion code under multiple docroots

Previously the code blew up trying to filename:join() a
{multiple_roots, ...} tuple to "head.frag.html". This fix
searches all the docroots for "head.frag.html" and includes
the contents if it exists. An error is logged only in case
there is no "head.frag.html" found in the first docroot.
Errors in other docroots are ignored.

Change-Id: I466f0c08b56b44b8b2f080d4b36d3c04ca7b2c3a
Reviewed-on: http://review.couchbase.org/59756
Tested-by: Anders Nygren <4f8bff4ce020de40cfee5db6ca7fd51acc06e5c4@couchbase.com>
Reviewed-by: Dave Finlay <7b10a9ed9f70e64c30b07607a671ae61c33f14c8@couchbase.com>
Tested-by: Dave Finlay <7b10a9ed9f70e64c30b07607a671ae61c33f14c8@couchbase.com>

ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server