Create nodepool_pip_virtualenv_symlink variable

Expose the ability for a user to create a symlink to a nodepool
virtualenv, this is usually needed when you start trying to version
different virtualenv installs.

Change-Id: Id77609f696dcd4083001a101fdd22ba1634f4617
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

defaults/main.yaml

@@ -68,6 +68,7 @@ nodepool_pip_name: nodepool
 # nodepool_pip_version:
 # nodepool_pip_virtualenv_python:
 # nodepool_pip_virtualenv:
+# nodepool_pip_virtualenv_symlink:
 
 nodepool_docker_name:
   - zuul/nodepool-builder

tasks/install/pip.yaml

@@ -22,3 +22,14 @@
     version: "{{ nodepool_pip_version|default(omit) }}"
     virtualenv_python: "{{ nodepool_pip_virtualenv_python|default(omit) }}"
     virtualenv: "{{ nodepool_pip_virtualenv|default(omit) }}"
+
+- name: Symlink nodepool virtualenv
+  become: true
+  file:
+    src: "{{ nodepool_pip_virtualenv }}"
+    dest: "{{ nodepool_pip_virtualenv_symlink }}"
+    state: link
+  when:
+    - nodepool_pip_virtualenv is defined
+    - nodepool_pip_virtualenv_symlink is defined
+    - nodepool_pip_virtualenv != nodepool_pip_virtualenv_symlink