Split python support into 2.7 and 3.0

[micheleorsi]

I am in a situation where I need only python 2.7, without python 3.0 and I would like to have the opportunity to install just one. As it is already possible with ruby

[OnixGH]

Also explained in this PR, python 3 is a dependency in the base image, so unfortunately at this level splitting doesn't make a real difference (removing it doesn't save space).