Reactivate watcher dashboard plugin in devstack/local.conf.controller
Reactivate watcher dashboard plugin in devstack/local.conf.controller

Since watcher dashboard can be sucessfully installed now by devstack,
we should enable this again. Many of us are get the local.conf from
here,so this change is necessary, we can enable watch dashboard plugin
by default.

Change-Id: Iad5081a97515b3f831d2f468dc514a942e6d3420

stackforge/watcher,openstack/watcher,stackforge/watcher,openstack/watcher