tests: Update URL for ansible-role-systemd_service

This fixes the following problem

failed: [localhost] (item={'name': u'systemd_service', 'src':
u'https://github.com/openstack/ansible-role-systemd_service', 'scm':
u'git', 'version': u'master'}) => {"changed": false, "failed": true,
"item": {"name": "systemd_service", "scm": "git", "src":
"https://github.com/openstack/ansible-role-systemd_service", "version":
"master"}, "msg": "Failed to set a new url
https://github.com/openstack/ansible-role-systemd_service for origin:
fatal: No such remote 'origin'\n"}

This happens because zuul automatically clones dependant projects using
the git.openstack.org URLs but here we used the github.com one and that
led to module failures.

Change-Id: Id9b0f45af9f9393cd63e6f60b37e55cf16d08631

tests/ansible-role-requirements.yml

@@ -55,6 +55,6 @@
   scm: git
   version: master
 - name: systemd_service
-  src: https://github.com/openstack/ansible-role-systemd_service
+  src: https://git.openstack.org/openstack/ansible-role-systemd_service
   scm: git
   version: master