Adds a thirsty=yes|no to the get_url module, such that if downloading a large…

Adds a thirsty=yes|no to the get_url module, such that if downloading a large file from the internet you can decide
whether to download it every time or not -- will replace only on change, or decide to not download.  The default
is thirsty=no which will not download every time by default.

CHANGELOG.md

@@ -19,6 +19,7 @@ Module changes:
 * new module boilerplate code to check for mutually required arguments, arguments required together, exclusive args
 * add pattern= as a paramter to the service module (for init scripts that don't do status, or do poor status)
 * various fixes to mysql & postresql modules
+* added a thirsty= option (boolean, default no) to the get_url module to decide to download the file every time or not
 
 Core changes:
 

library/get_url

@@ -136,11 +136,20 @@ def main():
         argument_spec = dict(
             url = dict(required=True),
             dest = dict(required=True),
+            thirsty = dict(default='no', choices=BOOLEANS)
         )
     )
 
     url  = module.params['url']
     dest = os.path.expanduser(module.params['dest'])
+    thirsty = module.boolean(module.params['thirsty'])
+
+    if not thirsty:
+        if os.path.exists(dest):
+            module.exit_json(msg="file already exists", changed=False)
+        if os.path.isdir(dest):
+            module.fail_json(msg="non-thirsty mode needs a filename for a destination, not a directory")
+
 
     # download to tmpsrc
     tmpsrc, info = url_get(module, url, dest)