In Pike[1], we introduced a user_group auto detection for haproxy.
The default user group name is auto-detected for any OS distribution
we support as a base for Amphorae.
user_group remained as an option for admins but was also
marked deprecated in Pike[2].
This patch removes that option altogether.
Story: 2003323
Task: 24357
[1] Ia8fede9d7da4709a48661d1fc595a16d04fcbfa9
[2] https://review.openstack.org/#/c/429398/45/octavia/common/config.py@175
Change-Id: Iddd4162674f116705d2b47062cbf7ca88f2677a6
(cherry picked from commit 100858fa79)
8d74497cf6