##### 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
Action:
      - "kms:Encrypt"
      - "kms:Decrypt"
      - "kms:ReEncrypt*"
      - "kms:GenerateDataKey*"
      - "kms:CreateGrant"
      - "kms:DescribeKey"
      Resource: "*"
      Condition:
        StringEquals:
          kms:CallerAccount: "111111111111"
          kms:ViaService: "ec2.ap-southeast-2.amazonaws.com"
    - Sid: "Allow direct access to key metadata to the account"
      Effect: "Allow"
      Principal:
        AWS: "arn:aws:iam::111111111111:root"
      Action:
      - "kms:Describe*"
      - "kms:Get*"
      - "kms:List*"
      - "kms:RevokeGrant"
      Resource: "*"
tags:
  description: dictionary of tags applied to the key
  type: dict
  returned: always
  sample:
    Name: myKey
    Purpose: protecting_stuff
grants: