Fix bug 1467

Do not raise exceptions in n2vc.libjuju.add_model() or n2vc.libjuju.destroy_model() functions

In general, functions should be as idempotent as possible. In this
particular case, executing the add_model() function several times with
the same arguments should just work. Same thing for destroy_model(). If
the model has already been destroyed, it should return and not give any
errors saying that the model doesn't exist.

Change-Id: I87e11ea0fe1b4063b2f89900fcc2bbf1f915b953
Signed-off-by: David Garcia <david.garcia@canonical.com>