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": {
        "swift": {
            "venv_tag": "14.2.1"
        }
    }
}

Change-Id: I54f6317ba1b82c29a2b64c59e64cc7a34a82e880

tasks/swift_install.yml

@@ -119,3 +119,10 @@
 - include: swift_pypy_setup.yml
   static: no
   when: swift_pypy_enabled | bool
+
+- name: Record the venv tag deployed
+  ini_file:
+    dest: "/etc/ansible/facts.d/openstack_ansible.fact"
+    section: swift
+    option: venv_tag
+    value: "{{ swift_venv_tag }}"