Fix bug 1448: minor fix in JujuModelWatcher.wait_for_model

Add an `application_ready` function in the Juju Watcher to check if all the
applications of the model are ready.  This function checks that not only
the application is ready, but also all the units of the application.

Additionally, I have added a stability check. After considering a model
as ready, we will wait for 10 seconds, and check again.

Change-Id: Icd8d88613416b75cfd241e7044dbeffa6cbc6a8e
Signed-off-by: David Garcia <david.garcia@canonical.com>
(cherry picked from commit c3441173)