Allow arbitrary named hook functions for Pre and Post Hook feature of Safe Deployments.

If you need to use the Pre or Post Hook feature of the [Safe Deployments](https://github.com/awslabs/serverless-application-model/blob/master/docs/safe_lambda_deployments.rst#pretraffic-posttraffic-hooks), you need to name the hook function with a prefix (CodeDeployHook_). This is not super clear and breaks the mental model customers have on naming CloudFormation resources.

Option 1:
To make this feature more flexible and allow customers to follow the naming schema's they wish, I am proposing to add a [Policy Template](https://github.com/awslabs/serverless-application-model/blob/master/docs/policy_templates.rst). This would be mostly the same IAM Role CodeDeploy needs that takes the hook functions in as a parameter.

Option 2:
SAM could create the CodeDeploy IAM Role and inject the Lambda Function arns into the policy. In the `DeploymentPreference` block, customers are already defining the functions for the pre and post hook.

Thoughts?