Use config from settings.py

This enables to use a settings.py in the base directory of the repo,
if no environment variable set.

dev.py

@@ -1,5 +1,11 @@
 from __future__ import unicode_literals
 from __future__ import absolute_import
+import os
+
+if 'PUPPETBOARD_SETTINGS' not in os.environ:
+    os.environ['PUPPETBOARD_SETTINGS'] = os.path.join(
+        os.getcwd(), 'settings.py'
+    )
 
 from puppetboard.app import app
 from puppetboard.default_settings import DEV_LISTEN_HOST, DEV_LISTEN_PORT