fetch idempotence test and deprecate validate_md5

Added an integration test for fetch module idempotence.  (Testing
that validate_checksum is doing what it's supposed to is harder as we'd
have to create a race condition with the downloaded data to trigger it.
Probably need to make that a unittest eventually).

Also give a deprecation message to the validate_md5 parameter so that we
can eventually get rid of it.

test/integration/targets/fetch/tasks/main.yml

@@ -25,6 +25,13 @@
 
 - debug: var=fetched
 
+- name: Assert that we fetched correctly
+  assert:
+    that:
+      - 'fetched["changed"] == True'
+      - 'fetched["checksum"] == "a94a8fe5ccb19ba61c4c0873d391e987982fbbd3"'
+      - 'fetched["remote_checksum"] == "a94a8fe5ccb19ba61c4c0873d391e987982fbbd3"'
+
 # TODO: check the become and non-become forms of fetch because in one form we'll do
 # the get method of the connection plugin and in the become case we'll use the
 # fetch module.
@@ -38,6 +45,16 @@
     that:
       'diff.stdout == ""'
 
+- name: fetch a second time to show idempotence
+  fetch: src={{ output_dir }}/orig dest={{ output_dir }}/fetched
+  register: fetched
+
+- name: Assert that the file was not fetched the second time
+  assert:
+    that:
+      - 'fetched["changed"] == False'
+      - 'fetched["checksum"] == "a94a8fe5ccb19ba61c4c0873d391e987982fbbd3"'
+
 - name: attempt to fetch a non-existent file - do not fail on missing
   fetch: src={{ output_dir }}/doesnotexist dest={{ output_dir }}/fetched
   register: fetch_missing_nofail