Factorize common test code
Factorize common test code

Add doc & comments

Change-Id: I0b0438798c2c6bde82f27784193424860e947fe8
Signed-off-by: Vincent Legoll <5d42a0e44e2b47374e182c98b792e43fa0a82956@idgrilles.fr>

openstack/cliff,dtroyer/cliff,openstack/cliff,dtroyer/cliff