Add some useful features to our flags

* No longer dies if there are unknown flags.
* Allows you to declare that you will use a flag from another file
* Allows you to import new flags at runtime and reparses the original
  arguments to fill them once they are accessed.

nova/tests/declare_flags.py

@@ -0,0 +1,5 @@
+from nova import flags
+
+FLAGS = flags.FLAGS
+
+flags.DEFINE_integer('answer', 42, 'test flag')