Add Dwellers as lxc provider

Summary:
Dwellers is a server for Docker / LXC / OpenShift capabilities.

As such, it's the host for the lxc mailserver container.

Reference: https://docs.saltstack.com/en/latest/topics/cloud/lxc.html

Ref T1196

Test Plan: Provide a lxc container

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1196

Differential Revision: https://devcentral.nasqueron.org/D978