Deploy across an entire web cluster

Adds deploy, undeploy, and redeployAll to webapp cluster entities (DWAC and ControlledDWAC).

Also fixes some issues noticed in how tasks are tracked and displayed.
I don't understand how this or `BlobStoreTest` works. They both use `LocalManagementContextForTests`, so will not have read `brooklyn.properties`. Yet the comment says a named location with credentials needs to be defined (presumably in brooklyn.properties). Or am I missing something about what `LocalManagementContextForTests` does?