[edxec] Add missing health checks to app service

[jmaupetit]

Purpose

For now, we don't have health checks implemented in the edxec app service.

Proposal

Add & configure Mozilla's Dockerflow 2019.5.0 (more recent releases dropped support for Django 1.8) in our ecommerce's fork and use health monitoring views as we usually do for other apps like Richie.

[jmaupetit]

Note that we will need to change health check URL to make it generic and compatible with the official ecommerce distribution: see https://github.com/edx/ecommerce/blob/496c053dd05b10eac1179c6303651ab316e2a715/ecommerce/core/views.py#L29

[jmaupetit]

As an alternative, instead of integrating Mozilla's Dockerflow, we may consider backporting the official health view from edx to our fork.