Update Getting Started requirements versions

These docs currently say we target ubuntu 10.04 and an eventlet version
that our requirements file does not allow. Update these versions.

Change-Id: I052b6561f88ec90f865454e426032f1baf4586c0
Mention Python 2.7 in Getting Started guide

Change-Id: I2992d1227f294d4cb1c6a8676137df44e231bd80