Whitelist external netaddr requirement

* tox.ini(testenv.install_command): Use the --allow-external and
--allow-insecure options so that pip 1.5 and later will assent to
retrieve the netaddr package even though it's not hosted on PyPI.
The --allow-insecure option is aliased to a clearer
--allow-unverified wording in 1.5, but the old form is being used to
avoid breaking users of 1.4.x and will be valid at least through
1.6.x according to comments in the pip source.

Partial-Bug: #1266513

Change-Id: I005e752c8cb9bca78b1ebe777eb5230e9bf05bd8
Merge "Whitelist external netaddr requirement"