Clean up calls to get_node()

This patch removes all direct calls to dbapi's get_node(). They are
replaced either by Node.get(), Node.get_by_id(), or Node.get_by_uuid()
calls.

Additional detail about the changes:

1) Break DBAPI get_node() into get_node_by_id() and get_node_by_uuid().
   Let's be explicit about which type we're passing. This allows
   Node.get_by_uuid() to work exactly how its named. Also, do not
   return an object from the DB API calls. The Node object itself will
   do the conversion.
2) Adds Node.get_by_id() to compliment the current Node.get_by_uuid()
   for places you are querying by the ID.
3) Adds Node.get() so that you can still have a single method to use
   to pass either and ID or a UUID.
4) Make all of the Node.get* calls set the 'context' so that you can
   later just node.save() without passing it. Doc strings have been
   updated to clarify what 'context' is for.

Change-Id: I995891b1645056a5b15c48e37b8bcd1e299ac468
Merge "Clean up calls to get_node()"