webadmin: Fix ClassCastException in cell admin

Fixes

07 nov. 2014 23:32:44 (httpd) [] RuntimeException while executing runnable org.dcache.cells.CellStub$1@135dac1f with executor com.google.common.util.concurrent.MoreExecutors$SameThreadExecutorService@12afd1d3
java.lang.ClassCastException: java.lang.String cannot be cast to diskCacheV111.vehicles.Message
        at org.dcache.cells.CellStub$1.run(CellStub.java:503) ~[dcache-core-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at com.google.common.util.concurrent.MoreExecutors$SameThreadExecutorService.execute(MoreExecutors.java:297) [guava-17.0.jar:na]
        at com.google.common.util.concurrent.ExecutionList.executeListener(ExecutionList.java:156) [guava-17.0.jar:na]
        at com.google.common.util.concurrent.ExecutionList.execute(ExecutionList.java:145) [guava-17.0.jar:na]
        at com.google.common.util.concurrent.AbstractFuture.set(AbstractFuture.java:185) [guava-17.0.jar:na]
        at org.dcache.cells.CellStub$CallbackFuture.set(CellStub.java:594) [dcache-core-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at org.dcache.cells.CellStub$CallbackFuture.answerArrived(CellStub.java:616) [dcache-core-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellNucleus$CallbackTask.innerRun(CellNucleus.java:1050) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellNucleus$AbstractNucleusTask.run(CellNucleus.java:1005) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at com.google.common.util.concurrent.MoreExecutors$SameThreadExecutorService.execute(MoreExecutors.java:297) [guava-17.0.jar:na]
        at dmg.cells.nucleus.CellNucleus.addToEventQueue(CellNucleus.java:813) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellGlue.sendMessage(CellGlue.java:557) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellNucleus.sendMessage(CellNucleus.java:371) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellAdapter.messageArrived(CellAdapter.java:831) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellNucleus$DeliverMessageTask.innerRun(CellNucleus.java:1095) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at dmg.cells.nucleus.CellNucleus$AbstractNucleusTask.run(CellNucleus.java:1005) [cells-2.12.0-SNAPSHOT.jar:2.12.0-SNAPSHOT]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) [na:1.8.0_20]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) [na:1.8.0_20]
        at java.lang.Thread.run(Thread.java:745) [na:1.8.0_20]

The problem was hidden by the use of a raw type and thus the compiler didn't
reject the code.

Target: trunk
Request: 2.11
Request: 2.10
Request: 2.9
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7478/
(cherry picked from commit 0b23cfca5c0faf2019dfcaa332875d00908c94f8)