jetty: update webdefault.xml to adhere to new specification

The current xml stanza concerning security constraints does
not observe the new requirement that all HTTP methods be
specified.

This patch updates the xml namespace to point to the newer xsd,
and adds the missing omission stanza.

Testing:

Before application of patch, the following type of warning appears in the log:

05 Nov 2014 11:45:13 (httpd) [] ServletContext@o.e.j.w.WebAppContext@1db9683d{/webadmin,file:/var/lib/dcache/httpd/webadmin/webapp/,STARTING}{/webadmin.war} has uncovered http methods for path: /

The patch silences this warning.

Target: master
Patch: https://rb.dcache.org/r/7488
Request: 2.11
Acked-by: Paul
Require-notes: no
Require-book: no