Change add_relation function in libjuju.py to accept saas

Before, app_1, endpoint1, app_2, endpoint2 parameters were needed.
Now we directly pass the endpoint names of the relation with the right format.
If "app:endpoint", it is understood that the endpoint it of a charm in the current model.
If "name", it is understood that the endpoint is a consumed endpoint from another model.

This function allows to cross-model-relate charms to external models

Change-Id: I84ab45ba735a1960ab742d9ec731c357ec1042c6
Signed-off-by: David Garcia <david.garcia@canonical.com>