Fix problem in openstack gates

Fix problem in openstack gates

For unkown reasons, when using the plain ppa, apt sets the Ubuntu
version wrongly in the openstack gates. The result is that all
gates using Ubuntu and ODL fail, for example in this patch:

https://review.openstack.org/#/c/553333/

Using this syntax instead, we force apt to get the correct Ubuntu
version and works, as tested here:

https://review.openstack.org/#/c/559065/

This syntax is valid from the point of view of ansible as it is shown
as an example in their documentation:

docs.ansible.com/ansible/latest/modules/apt_repository_module.html

Change-Id: I8440fa9714137f22e43aa8cf9291a0e82a3a02e1
Signed-off-by: Manuel Buil <mbuil@suse.com>