Deploy Sentry web application on PaaS Docker

Summary:
Sentry installations are grouped by “realms”, like our Jenkins installations.
Realm information is configurable in the new saas/ pillar entry.

The main wrapper script sentry allows to run command for any realm spawning
a new container and running sentry. It calls a local wrapper script with
correct links, and credentials information to run Docker container.

The standard image is configurable through environment. As such, there is no
current need to build a custom image.

Ref T1360

Test Plan: Tested on Equatower for Nasqueron Sentry

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1360

Differential Revision: https://devcentral.nasqueron.org/D2004