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.

Change-Id: I291684fa65279f0cb86f84e0d8e69bfa4b4087c0
Partial-Bug: #1266513
Merge "Whitelist external netaddr requirement"