Use updated get_url module checksum functionality

Starting in Ansible 2.0, the get_url [1] module provides the
ability for a checksum to be provided to the get_url module
which will be verified against the local destination file
and the task skipped if it matches.

[1] http://docs.ansible.com/ansible/get_url_module.html

This patch implements the use of this functionality.

The ability to ignore a venv download failure is also removed
as this is not necessary or desirable. It is better for the
download to fail and the playbook execution to stop immediately
so that the failure point is exposed.

Change-Id: I22304c2d6b019fae8e4234548c3811cb8b1eb02f

meta/main.yml

@@ -18,7 +18,7 @@ galaxy_info:
   description: Installation and setup of watcher
   company: Walmart
   license: Apache2
-  min_ansible_version: 1.9
+  min_ansible_version: 2.0
   platforms:
     - name: Ubuntu
       versions:

tasks/watcher_install.yml

@@ -39,42 +39,13 @@
   retries: 5
   delay: 2
 
-- name: Get local venv checksum
-  stat:
-    path: "/var/cache/{{ watcher_venv_download_url | basename }}"
-    get_md5: False
-  when:
-    - not watcher_developer_mode | bool
-  register: local_venv_stat
-
-- name: Get remote venv checksum
-  uri:
-    url: "{{ watcher_venv_download_url | replace('tgz', 'checksum') }}"
-    return_content: True
-  when:
-    - not watcher_developer_mode | bool
-  register: remote_venv_checksum
-
-# TODO: When project moves to ansible 2 we can pass this a sha256sum which will:
-#       a) allow us to remove force: yes
-#       b) allow the module to calculate the checksum of dest file which would
-#          result in file being downloaded only if provided and dest sha256sum
-#          checksums differ
 - name: Attempt venv download
   get_url:
     url: "{{ watcher_venv_download_url }}"
     dest: "/var/cache/{{ watcher_venv_download_url | basename }}"
-    force: yes
-  failed_when: false
-  register: get_venv
-  when:
-    - not watcher_developer_mode | bool
-    - (local_venv_stat.stat.exists == False or
-        {{ local_venv_stat.stat.checksum is defined and local_venv_stat.stat.checksum != remote_venv_checksum.content | trim }})
-
-- name: Set watcher get_venv fact
-  set_fact:
-    watcher_get_venv: "{{ get_venv }}"
+    checksum: "sha1:{{ lookup('url', watcher_venv_download_url | replace('tgz', 'checksum')) }}"
+  register: watcher_get_venv
+  when: not watcher_developer_mode | bool
 
 - name: Remove existing venv
   file:
@@ -117,8 +88,7 @@
   until: install_packages|success
   retries: 5
   delay: 2
-  when:
-    - watcher_get_venv | failed or watcher_developer_mode | bool
+  when: watcher_developer_mode | bool
   notify: Restart watcher services
 
 - name: Update virtualenv path
@@ -126,4 +96,4 @@
     virtualenv-tools --update-path=auto {{ watcher_bin | dirname }}
   when:
     - not watcher_developer_mode | bool
-    - watcher_get_venv | success
+    - watcher_get_venv | changed or watcher_venv_dir | changed