Adding contributing guide to tests/ structure
Adding contributing guide to tests/ structure

tests/ folder can be daunting for a newcomer to openstack-ansible.
This should clarify the uses of the files there, at least until
we implement our own cookie-cutter for roles.

Change-Id: I927500355ead4d8aae228f5df78e90cae8d2afb1

openstack/openstack-ansible,cloudnull/os-ansible-deployment,yanyao/openstack-ansible,BjoernT/openstack-ansible,robb-romans/openstack-ansible,bunchc/openstack-ansible,BjoernT/os-ansible-deployment,yanyao/openstack-ansible,cloudnull/os-ansible-deployment,openstack/openstack-ansible,BjoernT/os-ansible-deployment,cfarquhar/openstack-ansible,BjoernT/os-ansible-deployment,BjoernT/openstack-ansible,robb-romans/openstack-ansible,cfarquhar/openstack-ansible,stackforge/os-ansible-deployment,bunchc/openstack-ansible,stackforge/os-ansible-deployment