fix 1183: Delete native charms before destroying juju model

Destroying the juju model automatically destroys all applications and machines.
However it is not deleted if they are native charms in pending status.

Change-Id: I8b3580e201f1d403d36b6e07fa3947ea4a655683
Signed-off-by: tierno <alfonso.tiernosepulveda@telefonica.com>