(2.8) dcache-webadmin: change Jetty setting so .war is not unpacked

See http://rt.dcache.org/Ticket/Display.html?id=8489.

When upgrading from 2.6 to 2.10, there is a potential
for the webadmin directory timestamps to be more recent
than the timestamp of the /usr/share/dcache/classes/webapps/webadmin.war
file, which then causes Jetty to ignore the new war
and use the old directory. This subsequently provokes
jndi issues because the new httpd.batch file does not
pass through properties that the webadmin spring file
expects to find in the naming and directory interface.

The simplest solution is to turn off unpacking in
versions subsequent to 2.6.

Note that https://rb.dcache.org/r/7436/ does this for master.

Testing:

Installed 2.6.  Then updated to master (this patch) over it,
leaving the old webadmin directory intact.  Httpd confirmed
to start without issues, navigation through pages is normal.

Target: 2.8
Request: 2.7
Acked-by: Gerd
Require-book: no
Require-notes: yes

RELEASE NOTES:  The webadmin.war found in /usr/share/dcache/classes/webapps
is no longer unpacked (by default, /var/lib/dcache/httpd).  This
avoids potential conflicts between versions when update installs are run.