##### 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
argument_spec = ec2_argument_spec()
    argument_spec.update(
        dict(
            alias=dict(aliases=['key_alias']),
            policy_mode=dict(aliases=['mode'], choices=['grant', 'deny'], default='grant'),
            policy_role_name=dict(aliases=['role_name']),
            policy_role_arn=dict(aliases=['role_arn']),
            policy_grant_types=dict(aliases=['grant_types'], type='list'),
            policy_clean_invalid_entries=dict(aliases=['clean_invalid_entries'], type='bool', default=True),
            key_id=dict(aliases=['key_arn']),
            description=dict(),
            enabled=dict(type='bool', default=True),
            tags=dict(type='dict', default={}),
            purge_tags=dict(type='bool', default=False),
            grants=dict(type='list', default=[]),
            policy=dict(),
            purge_grants=dict(type='bool', default=False),