Fix releasenotes generation

Fix several problems, tox -e releasenotes did not run at all.

Fixes:
* Fix version import so that release note building works.
* There's no stable/mitaka branch for watcher, so remove the mitaka file,
  otherwise release notes build will fail.
* Add _static directory with empty placeholder for sphinx to run.

Change-Id: Ia4dd2f7a7743dfd7f7022be04552c401ade9953f

releasenotes/source/conf.py

@@ -12,6 +12,7 @@
 # serve to show the default.
 
 import sys, os
+from watcher import version as watcher_version
 
 # If extensions (or modules to document with autodoc) are in another directory,
 # add these directories to sys.path here. If the directory is relative to the