ec2_ami_find: set is_public to 'true' or 'false' (#19250)

* set is_public 'true' or 'false'

Despite being a boolean property, https://github.com/melta/boto/blob/master/boto/ec2/image.py:63 sets is_public = True only if the argument is passed in as the string 'true'. Likewise for False/'false'.

This is a workaround for that bug in boto2, to allow the documented parameter to work with valid yaml values.

fixes #5600

* only set is_public if true

lib/ansible/modules/cloud/amazon/ec2_ami_find.py

@@ -357,7 +357,7 @@ def main():
     if hypervisor:
         filter['hypervisor'] = hypervisor
     if is_public:
-        filter['is_public'] = is_public
+        filter['is_public'] = 'true'
     if name:
         filter['name'] = name
     if platform: