Fix docker_containers app_port/host test

Summary:
Instead of reading the pillar from paas/docker.sls,
with Salt Tower, we need to read every .sls files
in pillar/paas/docker/.

The new structure contains directly the services,
so we skip the "by node" step when reading the dictionary.

This is a follow-up for bbc48f9eb26d.

Ref T2123

Test Plan: (cd _tests && python3 -m unittest pillar/paas/test_docker.py)

Reviewers: DorianWinty

Maniphest Tasks: T2123

Differential Revision: https://devcentral.nasqueron.org/D3660