Check docker_container app_port/host

Summary:
If an entry under docker_containers has host defined, it needs also
to have app_port defined. The host key is used by nginx to determine
if a web configuration should be generated for this container.

This change offers an unit test to validate the pillar configuration,
and fixes the issue for exim container introduced in 3c52ac6a.

Test Plan: New config test provided

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2007