##### 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
module.fail_json_aws(e, msg="Failed to create initial key")
    key = get_key_details(connection, module, result['KeyId'])
    alias = module.params['alias']
    if not alias.startswith('alias/'):
        alias = 'alias/' + alias
    try:
        connection.create_alias(AliasName=alias, TargetKeyId=key['key_id'])
    except (botocore.exceptions.ClientError, botocore.exceptions.BotoCoreError) as e:
        module.fail_json_aws(e, msg="Failed to create alias")
    ensure_enabled_disabled(connection, module, key)
    for grant in module.params.get('grants'):
        grant_params = convert_grant_params(grant, key)
        try:
            connection.create_grant(**grant_params)
        except (botocore.exceptions.ClientError, botocore.exceptions.BotoCoreError) as e:
            module.fail_json_aws(e, msg="Failed to add grant to key")
    
    result = get_key_details(connection, module, key['key_id'])
    module.exit_json(changed=True, **result)
def delete_key(connection, module, key):
    changed = False
    if key['key_state'] != 'PendingDeletion':
        try:
            connection.schedule_key_deletion(KeyId=key['key_id'])
            changed = True
        except (botocore.exceptions.ClientError, botocore.exceptions.BotoCoreError) as e:
            module.fail_json_aws(e, msg="Failed to schedule key for deletion")
    result = get_key_details(connection, module, key['key_id'])
    module.exit_json(changed=changed, **result)
def get_arn_from_kms_alias(kms, aliasname):