LXD should also look in its environment for a proxy
LXD should also look in its environment for a proxy

In case people are running their LXDs behind a proxy, LXD should also look
in its environment for this information.

Closes #1264

Signed-off-by: Tycho Andersen <10c18fcb898ee6b17d6575d444b125d10b3f4d6a@canonical.com>

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