Add ability to specify IAM Role path.

[ndouglas]

What does this PR do?

See #874. This PR updates the CloudFormation template to support specifying a path for IAM roles, which is necessary in some contexts.

Motivation

Currently manually patching templates, which sux.

Testing Guidelines

Making the change in my patched copy of the template.

Additional Notes

I'm kind of an idiot, please be patient.

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog
• This PR passes the integration tests (ask a Datadog member to run the tests)
• This PR passes the unit tests
• This PR passes the installation tests (ask a Datadog member to run the tests)

[ge0Aja]

lgtm