curtin vmtests warns about host apt config

[Ryan Harper]

when running vmtests on digglett where we have an "invalid" apt file present in /etc/apt/apt.conf.d/

curtin complains

2016-11-09 10:16:22,033 - vmtests - INFO - Target Tarball: yakkety amd64/hwe-y 20161105
N: Ignoring file '50unattended-upgrades.ucf-dist' in directory '/etc/apt/apt.conf.d/' as it has an invalid filename extension

[Ryan Harper (raharper)]

From Scott:

http://paste.ubuntu.com/23451558/

=== modified file 'tools/launch'
--- tools/launch 2016-07-15 16:14:53 +0000
+++ tools/launch 2016-11-09 16:23:17 +0000
@@ -235,16 +235,17 @@

 find_apt_proxy() {
     # pick an apt proxy for the guest
- local out=""
+ local out="" r=""

     # if user set uncommon 'apt_proxy', then trust it
     [ -n "$apt_proxy" ] && echo "$apt_proxy" && return 0

     # see if the host has an apt proxy configured, and use it
     if command -v apt-config >/dev/null 2>&1; then
- out=$(apt-config shell x Acquire::HTTP::Proxy) &&
+ out=$(apt-config shell x Acquire::HTTP::Proxy 2>&1) ||
+ { r=$?; error "apt-config shell failed [$r]: $out"; return $r; }
         out=$(sh -c 'eval $1 && echo $x' -- "$out") && [ -n "$out" ] &&
- echo "$out" && return
+ echo "$out" && return
     fi

     return 1