elb_application_lb purge rules option (#43113)

* Add parameter to keep elb rules

Does not purge elb rules. This is usefull if running the elb_application_lb
 role and there is the desire to keep existing rules.

* Change variable name keep_rules to purge_rules

The descriptor purge has been used in the past.

* Changed default for purge_rules

Default is purge_rules. This is how the module has functioned previously. This change maintains
 the previous behavior.

* Add integration test for purge_rules flag

* Change wording of test task

* Fix merge conflcit

* Changed default for purge_rules

Default is purge_rules. This is how the module has functioned previously. This change maintains
 the previous behavior.

* merge conflcit

* Change wording of test task

* Add purge_rules option to test

* Change test description wording

* Expand purge_rules documentation

* Clarifies documentation for purge_rules option

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

@@ -119,6 +119,12 @@ options:
     description:
       - The time in seconds to use in conjunction with I(wait).
     version_added: 2.6
+  purge_rules:
+    description:
+      - When set to no, keep the existing load balancer rules in place. Will modify and add, but will not delete.
+    default: yes
+    type: bool
+    version_added: 2.7
 extends_documentation_fragment:
     - aws
     - ec2
@@ -444,10 +450,11 @@ def create_or_update_elb(elb_obj):
             rules_to_add, rules_to_modify, rules_to_delete = rules_obj.compare_rules()
 
             # Delete rules
-            for rule in rules_to_delete:
-                rule_obj = ELBListenerRule(elb_obj.connection, elb_obj.module, {'RuleArn': rule}, rules_obj.listener_arn)
-                rule_obj.delete()
-                elb_obj.changed = True
+            if elb_obj.module.params['purge_rules']:
+                for rule in rules_to_delete:
+                    rule_obj = ELBListenerRule(elb_obj.connection, elb_obj.module, {'RuleArn': rule}, rules_obj.listener_arn)
+                    rule_obj.delete()
+                    elb_obj.changed = True
 
             # Add rules
             for rule in rules_to_add:
@@ -524,7 +531,8 @@ def main():
             state=dict(choices=['present', 'absent'], type='str'),
             tags=dict(type='dict'),
             wait_timeout=dict(type='int'),
-            wait=dict(default=False, type='bool')
+            wait=dict(default=False, type='bool'),
+            purge_rules=dict(default=True, type='bool')
         )
     )