slipstream & LAL OpenStack infrastructure

Due to some limitation on current slipstream release (v2.23.2), only v3.2+ version of OpenStack is compatible with LAL OpenStack infrastructure.

stable version of slipstream is available @ https://slipstream.stratuslab.eu
v3.2 (release candidate) version of slipstream is available @ https://slipstream-v3.lal.in2p3.fr