When force=yes, get_url should always download the specified file

This is accomplished by not setting the If-Modified-Since header,
and setting "cache-control: no-cache" instead. Note that if the
file content has not changed, the module will still report that
changed=false, as the md5's of the tmp file and existing file are
compared before swapping

Fixes ansible#5104

library/network/get_url

@@ -130,7 +130,7 @@ def url_filename(url):
         return 'index.html'
     return fn
 
-def url_do_get(module, url, dest, use_proxy, last_mod_time):
+def url_do_get(module, url, dest, use_proxy, last_mod_time, force):
     """
     Get url and return request and info
     Credits: http://stackoverflow.com/questions/7006574/how-to-download-file-from-ftp
@@ -176,9 +176,11 @@ def url_do_get(module, url, dest, use_proxy, last_mod_time):
     request = urllib2.Request(url)
     request.add_header('User-agent', USERAGENT)
 
-    if last_mod_time:
+    if last_mod_time and not force:
         tstamp = last_mod_time.strftime('%a, %d %b %Y %H:%M:%S +0000')
         request.add_header('If-Modified-Since', tstamp)
+    else:
+        request.add_header('cache-control', 'no-cache')
 
     try:
         r = urllib2.urlopen(request)
@@ -194,16 +196,15 @@ def url_do_get(module, url, dest, use_proxy, last_mod_time):
 
     return r, info
 
-def url_get(module, url, dest, use_proxy, last_mod_time):
+def url_get(module, url, dest, use_proxy, last_mod_time, force):
     """
     Download data from the url and store in a temporary file.
 
     Return (tempfile, info about the request)
     """
 
-    req, info = url_do_get(module, url, dest, use_proxy, last_mod_time)
+    req, info = url_do_get(module, url, dest, use_proxy, last_mod_time, force)
 
-    # TODO: should really handle 304, but how? src file could exist (and be newer) but empty
     if info['status'] == 304:
         module.exit_json(url=url, dest=dest, changed=False, msg=info.get('msg', ''))
 
@@ -283,7 +284,7 @@ def main():
         last_mod_time = datetime.datetime.utcfromtimestamp(mtime)
 
     # download to tmpsrc
-    tmpsrc, info = url_get(module, url, dest, use_proxy, last_mod_time)
+    tmpsrc, info = url_get(module, url, dest, use_proxy, last_mod_time, force)
 
     # Now the request has completed, we can finally generate the final
     # destination file name from the info dict.