Minor improvements to libjuju.py

- Increase default timeout to connect to the controller
- Minor fix in destroy_model function
- Fix unit tests. Now they run faster :-)

Change-Id: Ia2cbcb3e052fe91f4e712ea572ecf444b1d5894f
Signed-off-by: David Garcia <david.garcia@canonical.com>
(cherry picked from commit 435b8641)