Pip 8.0.0, released a few hours ago, tries to uninstall deb-installed
python modules that it wants to upgrade, and always fails when doing so.
The effect is that configuration/requirements.txt cannot be fulfilled,
notably ansible 1.9.3-edx, which requires an upgrade to pycrypto.

This forces the installation of pip to the latest one prior to 8.0.0,
7.1.2.

It also freezes setuptools and virtualenv, so that they should all match
the current versions required on
playbooks/roles/common_vars/defaults/main.yml:common_pip_pkgs.

1. The libxmlsec1-dev package is missing from the script (but exists on the Wiki page), which causes installation fails.
2. G++ 4.8 is required to build v8 engine which is required by nodejs used by edx-insights. But the offical Ubuntu 12.04 only provides gcc 4.6.3, so we should set ppa's tool chain and install gcc/g++ 4.8 before actual installation.

This is to deal with the issue pointed out in #2036