AWS FMS: SecurityServicePolicyData rendering entirely as JSON

By [AWS CloudFormation documentation](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-fms-policy.html#aws-resource-fms-policy--examples), the YAML representation of the SecurityServicePolicyData has two child nodes... _Type_ and _ManagedServiceData_. They are represented in YAML as follows:

```yaml
PolicyWAFv2:
Type: AWS::FMS::Policy
Properties:
ExcludeResourceTags: false
PolicyName: Policy
RemediationEnabled: false
ResourceType: AWS::ElasticLoadBalancingV2::LoadBalancer
SecurityServicePolicyData:
Type: WAFV2
ManagedServiceData: !Sub '{"type":"WAFV2",
"preProcessRuleGroups":[{
"ruleGroupType":"RuleGroup",
"ruleGroupArn":"${RuleGroup.Arn}",
"overrideAction":{"type":"NONE"}}],
"postProcessRuleGroups":[],
"defaultAction":{"type":"BLOCK"}}'
```

The _ManagedServiceData_ property is the only item of the node that is supposed to be JSON based, but in the [FMS::Policy troposphere object](https://github.com/cloudtools/troposphere/blob/26d0663e7ad5ac632a5aeaee34d66ded8195a116/troposphere/fms.py#L31), the entire thing is coded/output as JSON, which results in outputs like this:

```yaml
BaseProtectionsIntPolicy:
Properties:
DeleteAllPolicyResources: true
ExcludeResourceTags: false
IncludeMap:
ACCOUNT:
- '123456789012'
ORGUNIT: []
PolicyName: Base-Protections-Int-Policy
RemediationEnabled: false
ResourceType: ResourceTypeList
ResourceTypeList:
- AWS::ElasticLoadBalancingV2::LoadBalancer
- AWS::ApiGateway::Stage
SecurityServicePolicyData: >-
{"type": "WAFV2", "ManagedServiceData": "{\"type\": \"WAFV2\", \"preProcessRuleGroups\":
[{\"ruleGroupArn\": null, \"overrideAction\": {\"type\": \"COUNT\"}, \"managedRuleGroupIdentifier\":
{\"version\": null, \"vendorName\": \"AWS\", \"managedRuleGroupName\": \"AWSManagedRulesCommonRuleSet\"},
\"ruleGroupType\": \"ManagedRuleGroup\"}, {\"ruleGroupArn\": null, \"overrideAction\":
{\"type\": \"COUNT\"}, \"managedRuleGroupIdentifier\": {\"version\": null,
\"vendorName\": \"AWS\", \"managedRuleGroupName\": \"AWSManagedRulesAmazonIpReputationList\"},
\"ruleGroupType\": \"ManagedRuleGroup\"}, {\"ruleGroupArn\": \"${BaseIntRuleGroup.Arn}\",
\"overrideAction\": {\"type\": \"NONE\"}, \"ruleGroupType\": \"RuleGroup\"}],
\"postProcessRuleGroups\": [], \"defaultAction\": {\"type\": \"ALLOW\"}}"}
Type: AWS::FMS::Policy
```

Furthermore, attempting to add a troposphere Sub() object around the _ManagedServiceData_ string results in an error that the Sub() object is not JSON Serializable.

I propose that an additional helper object be defined in FMS.py that allows further breakout of these items and moves the json_checker validation to the _ManagedServiceData_ definition under the new object.

I'm curious to know if anyone has this working as is.

Thanks!