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": {
        "sahara": {
            "venv_tag": "14.2.1"
        }
    }
}

Change-Id: I43a8895d111de16386a58acd30cc7a5e2c8259db

tasks/sahara_install.yml

@@ -113,3 +113,10 @@
   command: >
     virtualenv-tools --update-path=auto --reinitialize {{ sahara_bin | dirname }}
   when: sahara_get_venv | changed
+
+- name: Record the venv tag deployed
+  ini_file:
+    dest: "/etc/ansible/facts.d/openstack_ansible.fact"
+    section: sahara
+    option: venv_tag
+    value: "{{ sahara_venv_tag }}"