lxc autopkgtest fails if http_proxy not set

[Seth Forshee]

debian/tests/exercise fails if http_proxy is not set, e.g.:

https://objectstorage.prodstack4-5.canonical.com/v1/AUTH_77e2ada1e7a84929a74ba3b87153c0ac/autopkgtest-zesty/zesty/amd64/l/lxc/20170320_194539_28897@/log.gz

Since the script uses 'set -e' the expression 'if [ -n "${http_proxy}" ]' causes the script to terminate when http_proxy isn't set.

[Stéphane Graber (stgraber)]

Packaging branches updated, will be in the next round of bugfix releases.