dcache-webadmin: fix page update

This patch addresses several RT tickets stemming from the same problem, one which has already been corrected for the CellServices page (see http://rb.dcache.org/r/5807).

This has to do with the way Wicket PropertyModels were provided for pages which maintain lists which need to be updated through the service layer.  In most of these cases, the list was set in the constructor but was never updated during page/form refresh.  The changes adopted here follow the same general pattern of replacing the field accessed by the model with a getter method which also takes care of calling the service to update the collection in question.

Testing:

Redeployed and checked updating by enabling/disabling pools.

Target: master
Request: 2.7
Patch: http://rb.dcache.org/r/6080
Require-notes: yes
Require-book: no
Bug: http://rt.dcache.org/Ticket/Display.html?id=7929
Bug: http://rt.dcache.org/Ticket/Display.html?id=7964
Bug: http://rt.dcache.org/Ticket/Display.html?id=7983
Acked-by: Gerd
Acked-by: Tigran

RELEASE NOTES:
Fixes an issue with some of the tables in the webadmin service which were not refreshing.  Update correctly occurs now on reload, resubmission of the form through link clicking