Create zuul/web/static on demand

We generate the static web build into zuul/web/static which gets
cleaned by yarn before the build. This directory must be existing
before running the yarn build. In order to guarantee that it exists we
currently have the file zuul/web/static/.keep under version
control. This problem with this is that every time the yarn build is
done (e.g. re-creating the tox environment) this file is deleted. This
file deletion is often added to commits accidentally. We can avoid
needing this file if we create the target dir on demand.

Change-Id: I6e94316a89373af1cbfc0abcfc403b54e33046a3

tools/pip.sh

@@ -32,6 +32,7 @@ then
 fi
 if [[ ! -f zuul/web/static/status.html ]]
 then
+    mkdir -p zuul/web/static
     pushd web/
         yarn install
         yarn build

zuul/_setup_hook.py

@@ -29,6 +29,7 @@ def _build_javascript():
         if r:
             raise RuntimeError("Yarn install failed")
     if not os.path.exists('zuul/web/static/index.html'):
+        os.makedirs('zuul/web/static', exist_ok=True)
         r = subprocess.Popen(['yarn', 'build'], cwd="web/").wait()
         if r:
             raise RuntimeError("Yarn build failed")