Swift: a server process is terminated ungracefully when stopping its container

Description
===========
Currently SIGTERM is widely used to stop processes inside containers.
However, sending SIGTERM to a swift-*-server process terminates the process ungracefully, and this can cause interruption of on-going API requests.
For swift*-server process, we should use SIGHUP so that the process stops gracefully after it completes all on-going API requests.

Steps to reproduce
==================
* Stop swift_proxy container
 $ sudo systemctl stop tripleo_swift_proxy

* Check swift.log

Expected result
===============
swift-proxy-server process receives SIGHUP and stops gracefully

Actual result
=============
swift-proxy-server process receives SIGTERM and stops immediately
Jul 18 23:39:25 controller-0 proxy-server[6828]: SIGTERM received
Jul 18 23:39:25 controller-0 proxy-server[6828]: Exited (7)

Environment
===========
The issue was initially found in stable/train deployment

Logs & Configs
==============
N/A