Passing config for flat type network

Add support to passing the flat_network configuration. User can
either passing the physical network name, or use the physical
network definition

Change-Id: Ie42679f207eb14620883778314f74abf378b5cbc
Includes a validation to disable creating initial networks for Neutron

Neutron external and internal networks are created by default
This commit let developers by configuration to decide if those
networks needs to be created. This is needed to test Neutron DVR
in a distributed Multi-node environment

Change-Id: I17d891d072f189925676b4557094cde1c7a71579
Closes-Bug: 1389288
Drop PIP_USE_MIRRORS environment variable

The PIP_USE_MIRRORS does not do anything else than adding
 the --use-mirrors option to the pip command line.

The --use-mirrors is deprecated since pip-1.5,
and does not do anything else than printing a warning message.

For using alternate pypi index url this option is not required and
not sufficient. The current way for using alternate mirrors
is to defining them in the .pip/pip.conf either manually (before
devstack), or by using the PYPI_ALTERNATIVE_URL and PYPI_OVERRIDE
environment variables.

Change-Id: Ia33e783360e5661c2ef03b77e9f7af32b2633f2f
Merge "Passing config for flat type network"
Merge "Drop PIP_USE_MIRRORS environment variable"
Merge "Includes a validation to disable creating initial networks for Neutron"