install-server: pin rspec < 3 and serverspec

Ensure we have the good versions to run serverspec and rspec without
warning when running functionnal testing on install-server node.

Co-Authored-By: Sébastien Badia <sebastien.badia@enovance.com>
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>