fix races during removing of a cluster

fixes:
ava.lang.NullPointerException
null
lcmc.data.Host.stopDrbdStatus(Host.java:1238)
lcmc.data.Host.waitOnDrbdStatus(Host.java:1252)
lcmc.gui.ClusterBrowser.startDrbdStatus(ClusterBrowser.java:1126)
lcmc.gui.ClusterBrowser$9.run(ClusterBrowser.java:928)