Read flatter docker_networks pillar

Summary:
The docker_network pillar contains directly the networks,
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

Reviewed By: DorianWinty

Maniphest Tasks: T2123, T1817

Differential Revision: https://devcentral.nasqueron.org/D3662