Add venv_tag local fact

In order to make it easier to detect the currently deployed
venv for a service, and therefore allow smarter decisions
for things like upgrading, we implement the venv tag as a
local fact.

The file used to store facts will be the same for all
OpenStack services, with each service using its own section.

Example:

"ansible_local": {
    "openstack_ansible": {
        "neutron": {
            "venv_tag": "14.2.1"
        }
    }
}

Change-Id: Ib5697e47a44284977168faf4bc2d6a21f4703268

tasks/neutron_install.yml

@@ -139,3 +139,10 @@
   retries: 5
   delay: 2
   notify: Restart neutron services
+
+- name: Record the venv tag deployed
+  ini_file:
+    dest: "/etc/ansible/facts.d/openstack_ansible.fact"
+    section: neutron
+    option: venv_tag
+    value: "{{ neutron_venv_tag }}"