##### 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
sample: "2017-04-18T15:12:08.551000+10:00"
description:
  description: Description of the key
  type: str
  returned: always
  sample: "My Key for Protecting important stuff"
enabled:
  description: Whether the key is enabled. True if C(KeyState) is true.
  type: str
  returned: always
  sample: false
aliases:
  description: list of aliases associated with the key
  type: list
  returned: always
  sample:
    - aws/acm
    - aws/ebs
policies:
  description: list of policy documents for the keys. Empty when access is denied even if there are policies.
  type: list
  returned: always
  sample:
    Version: "2012-10-17"
    Id: "auto-ebs-2"
    Statement:
    - Sid: "Allow access through EBS for all principals in the account that are authorized to use EBS"
      Effect: "Allow"
      Principal:
        AWS: "*"