Add task_exec and task role policy attachments

[sodre]

what

• Creates two variables to accept a list of policies to attach to the generated task and task execution roles
• The policies are only attached if the the roles were created by this module.

why

• When creating services that reference outside secrets it would be easier to pass the required policy during service creation.

references

• This is related to Add example to add iam policies to task role #67 and Add example to add iam policies to task role #68
• Supersedes and closes Update documentation links #86

[joe-niland]

@sodre the title starts with WIP. Ready to review?

[sodre]

@joe-niland, this should be read for review.

[Nuru]

/test all

[Nuru]

/test all