If you purge lxd from a system, then all lxd-related system files should also be removed. But /etc/default/lxd-bridge and /etc/default/lxd are kept, and *will* be used on a new install.
ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: lxd 2.1-0ubuntu1~ubuntu16.04.1~ppa1 [origin: LP-PPA-ubuntu-lxc-lxd-stable]
ProcVersionSignature: Ubuntu 4.4.0-34.53+jj51-generic 4.4.15
Uname: Linux 4.4.0-34-generic x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Aug 18 11:40:49 2016
InstallationDate: Installed on 2016-02-08 (192 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Alpha amd64 (20160205)
SourcePackage: lxd
UpgradeStatus: No upgrade log present (probably fresh install)
This bug was fixed in the package lxd - 2.1-0ubuntu2
---------------
lxd (2.1-0ubuntu2) yakkety; urgency=medium
* Properly handle purge. (LP: #1614621)
* Configure system-wide dnsmasq not to touch lxdbr0. (LP: #1613820)
* Stop lxd.socket on package remove. (LP: #1577001)
-- Stéphane Graber <email address hidden> Mon, 29 Aug 2016 17:29:27 -0400