Fix for lookup and get_url occuring in different places

We use an SSH bastion host which we do our deployment through. The
deployment host doesn't have direct access to the same network as the
host. As a result the venv local checksum lookup fails.

I have described this here:

https://bugs.launchpad.net/openstack-ansible/+bug/1689283

This is a simple fix for this problem, assuming everything is good it
will need repeating in multiple places in the code base.

Change-Id: I290fd1e5763bc89028bb9207dbd0bba33bacaa06
(cherry picked from commit 4e4600ef93)

tasks/watcher_install.yml

@@ -39,11 +39,18 @@
   retries: 5
   delay: 2
 
+- name: Retrieve checksum for venv download
+  uri:
+    url: "{{ watcher_venv_download_url | replace('tgz', 'checksum') }}"
+    return_content: yes
+  register: watcher_venv_checksum
+  when: watcher_venv_download | bool
+
 - name: Attempt venv download
   get_url:
     url: "{{ watcher_venv_download_url }}"
     dest: "/var/cache/{{ watcher_venv_download_url | basename }}"
-    checksum: "sha1:{{ lookup('url', watcher_venv_download_url | replace('tgz', 'checksum')) }}"
+    checksum: "sha1:{{ watcher_venv_checksum.content | trim }}"
   register: watcher_get_venv
   when: watcher_venv_download | bool