Removed http proxy environment variable so that httpretty can work

If test environment is located behind a http proxy, tox and pip
require to set http proxy environment variable.
However httpretty mock library does NOT work as expected if http proxy
is set.
Therefor code to remove the http proxy environment variable is added
into the fixture.

Change-Id: Ibd3bb833774b1362e628725603b9b274b09375bd
Closes-Bug: #1403046
Merge "Removed http proxy environment variable so that httpretty can work"