##### 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
version_added: 2.8
author:
  - Ted Timmons (@tedder)
  - Will Thames (@willthames)
extends_documentation_fragment:
- aws
- ec2
'''
EXAMPLES = '''
# Managing the KMS IAM Policy via policy_mode and policy_grant_types is fragile
# and has been deprecated in favour of the policy option.
- name: grant user-style access to production secrets
  aws_kms:
  args:
    alias: "alias/my_production_secrets"
    policy_mode: grant
    policy_role_name: "prod-appServerRole-1R5AQG2BSEL6L"
    policy_grant_types: "role,role grant"
- name: remove access to production secrets from role
  aws_kms:
  args:
    alias: "alias/my_production_secrets"
    policy_mode: deny
    policy_role_name: "prod-appServerRole-1R5AQG2BSEL6L"
# Create a new KMS key
- aws_kms:
    alias: mykey
    tags:
      Name: myKey
      Purpose: protect_stuff