##### SUMMARY

If you attempt to modify the policy on an existing KMS CMK using aws_kms's "policy" option this doesn't appear to get updated.

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
lib/ansible/modules/cloud/amazon/aws_kms.py

##### ANSIBLE VERSION

```
ansible 2.8.1
  config file = /home/mchappel/.ansible.cfg
  configured module search path = [u'/home/mchappel/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/lib/python2.7/site-packages/ansible
  executable location = /usr/bin/ansible
  python version = 2.7.5 (default, Jun 11 2019, 12:19:05) [GCC 4.8.5 20150623 (Red Hat 4.8.5-36)]
```

##### CONFIGURATION

```
DEFAULT_CALLBACK_WHITELIST(/home/mchappel/.ansible.cfg) = [u'timer', u'profile_tasks']
HOST_KEY_CHECKING(/home/mchappel/.ansible.cfg) = False
RETRY_FILES_ENABLED(/home/mchappel/.ansible.cfg) = False
```

##### OS / ENVIRONMENT
RHEL7 + AWS


##### STEPS TO REPRODUCE

- Create KMS CMK
- Attempt to update policy using the 'policy' option on aws_kms

```
    - name: 'Update policy on KMS Keys'
      aws_kms:
        key_id: '{{ kms_key.id }}'
        alias: '{{ kms_key.alias | default(omit) }}'
        policy: ' {{ kms_current_policy | to_json }}'
```

##### EXPECTED RESULTS
KMS Policy is updated

##### ACTUAL RESULTS
KMS Policy stays the same
return connection.get_key_policy(KeyId=key_id, PolicyName=policy_name)
def get_kms_tags(connection, module, key_id):
    
    
    kwargs = {}
    tags = []
    more = True
    while more:
        try:
            tag_response = get_kms_tags_with_backoff(connection, key_id, **kwargs)
            tags.extend(tag_response['Tags'])
        except is_boto3_error_code('AccessDeniedException'):
            tag_response = {}
        except (botocore.exceptions.ClientError, botocore.exceptions.BotoCoreError) as e:  
            module.fail_json_aws(e, msg="Failed to obtain key tags")
        if tag_response.get('NextMarker'):
            kwargs['Marker'] = tag_response['NextMarker']
        else:
            more = False
    return tags
def get_kms_policies(connection, module, key_id):
    try:
        policies = list_key_policies_with_backoff(connection, key_id)['PolicyNames']
        return [get_key_policy_with_backoff(connection, key_id, policy)['Policy'] for
                policy in policies]
    except is_boto3_error_code('AccessDeniedException'):
        return []
    except (botocore.exceptions.ClientError, botocore.exceptions.BotoCoreError) as e:  
        module.fail_json_aws(e, msg="Failed to obtain key policies")
def key_matches_filter(key, filtr):