deploy each itest in its own model

[PietroPasotti]

    I think deploying all itests in the same model is not a justification but rather something to avoid.

The introduction of deploy_traefik_if_not_deployed and deploy_charm_if_not_deployed seems to me like an antipattern that should be avoided.

In the past we were trying out reset but it wasn't very stable. If we add a preserve keyword to reset perhaps we could try again.

Originally posted by @sed-i in #103 (comment)

[sed-i]

Related: juju/python-libjuju#776

[dstathis]

This is old and we are making plans for this sort of thing across all charms.