feature: Add ability to specify IAM role

[ShaunEdiger]

Steps to reproduce

1. Create a Serverless Framework project with at least one function
2. Specify a role for each function by providing its logical id under the role property (of each lambda function)
3. Introduce serverless-plugin-healthcheck

Expected Result

I would like to specify the IAM role used by the generated healthcheck lambda function.

Actual Result

I can't specify the IAM role used by the generated healthcheck lambda function because it's not exposed by the plugin. A new IAM role appears in the CloudFormation stack. I can add statements via Serverless Framework's iamRoleStatements feature, but that isn't the same as specifying a role by its id.

Why is this a problem?

Each lambda function or classes of lambda functions may have their own IAM role that fulfills unique security requirements. I might not want to fallback to the IAM role generated by Serverless Framework, and I might not want it to be shared with other lambda functions.

Workaround
Allow Serverless Framework to generate the default IAM role and customize it somewhat by adding statements to it using iamRoleStatements. This only works assuming all other declared lambda functions already have roles, that is to say that the default IAM policy isn't already in use, or by assuming that it's acceptable for different classes of lambda functions to share the same IAM role.

What's going on

If Serverless Framework detects that all lambda functions have an IAM role assigned, it won't take action. If it detects functions without roles, it will create a role for the role-less lambda functions to use (and share if there multiple role-less lambda functions). In our specific case, the lambda function generated by the plugin has no IAM role and will be treated just like any other lambda function without an IAM role.

What this pull request does

Exposes an option to specify the IAM role used for the healthcheck lambda function, including documentation.