Move the pip_install to 'stack' stage
Move the pip_install to 'stack' stage

Nothing guarantees that pip is installed in 'pre-install' stage. Much
better to move the pip_install calls to 'stack'

Change-Id: Ibbbe72073bff12f2be5a36d2aae6ec384b9924f3

JoeMido/networking-midonet,yamt/networking-midonet,yamt/networking-midonet,JoeMido/networking-midonet