test/test_http.py: also backup/restore HTTP proxy settings
test/test_http.py: also backup/restore HTTP proxy settings

The HttpLayerTestCase class relies on the LibcloudConnection class -
which honors https_proxy and http_proxy variables.

Since the tests expect connections to the loopback interface (e.g.,
127.0.0.1), we need to clear the http_proxy and https_proxy
variables before running the tests. Later on, we need to restore the
variables just like we did in test/test_connection.py

Signed-off-by: Olivier Gayot <177cb43a9a0ccab7ae6d0e6a74e79a9be7a5415d@canonical.com>

apache/libcloud,apache/libcloud,apache/libcloud