Remove unused variable in agent._get_interfaces()

Change-Id: I34f1d0d12fe949e99722c9315480ac6ae45ff683
dev-quickstart.rst update to add required packages

* added unspecifed required package "python-tox" for Ubuntu/Debian,
  and removed "py26" from "tox.ini" so that "tox" command doesn't
  invoke the tests in Python 2.6 environment.

Change-Id: I57df22a54fc08f2bb53a17d13b70706952815871
Closes-Bug: #1392304
Merge "dev-quickstart.rst update to add required packages"
Merge "Remove unused variable in agent._get_interfaces()"