refactor the ara database schema

this is a major rewrite of the database schema and the ansible
callback module (which means that nothing else will work with this
commit at the moment).  This commit makes the following changes:

- We move some default values into a separate ara.defaults module just
  to keep things tidy.

- We always create ~/.ara (or wherever `ARA_DIR` points).  This
  simplifies working with sqlite: if you're using the default database
  it Just Works, and if you want to put it someplace else it's your
  job to make sure the destination directory exists.

- We make a number of column type changes in the database, so that
  dates are recorded as datetime types, and booleans are booleans, and
  we replace db.String with db.Text (because db.String gets converted
  into a MySQL VARCHAR, which requires a length).

- We introduce a new TaskResult entity.  This lets us track duration
  for an entire task vs. the time it takes on each individual host.
  It also avoids repeating information for host that is global to the
  task (like the name, action, path, line number, etc).

- We isolate all knowledge of primary keys to the ara.models module.

- We stop storing "duration" in the database, because this can be
  calculated.  Most of the database entities inherit from a
  `TimedEntity` mixin that adds the `duration` property which will
  perform this calculation automatically.

- We introduce a 'commit' decorator to avoid lots of boilerplate code.

- Database configuration is now always provided as a SQLAlchemy URI.
  These changes have been tested with both sqlite and mysql.

requirements.txt

@@ -1,3 +1,4 @@
 Flask
 Flask-SQLAlchemy
-pbr>=1.6
+pbr>=1.6
+decorator