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": {
        "barbican": {
            "venv_tag": "14.2.1"
        }
    }
}

Change-Id: I086c66d720ead341c6186462bdcd1ecf5fd00b34

tasks/barbican_install.yml

@@ -102,3 +102,10 @@
   command: >
     virtualenv-tools --update-path=auto --reinitialize {{ barbican_bin | dirname }}
   when: barbican_get_venv | changed
+
+- name: Record the venv tag deployed
+  ini_file:
+    dest: "/etc/ansible/facts.d/openstack_ansible.fact"
+    section: barbican
+    option: venv_tag
+    value: "{{ barbican_venv_tag }}"