[r=axwalk],[bug=1322136] utils: clear $*_proxy in tests
[r=axwalk],[bug=1322136] utils: clear $*_proxy in tests

Clear $http_proxy, $HTTP_PROXY,
$https_proxy, and $HTTPS_PROXY
when running the tests.

As of Go 1.3, net/http will use
a sync.Once to get the proxy
env var values; thus, we clear
these variables at program
initialisation.

Fixes lp:1322136

https://codereview.appspot.com/98450046/
dimitern/utils,rogpeppe/juju-utils