Undefined method managed_policy_arn for iam_user resource

[erran-r7]

We should define managed_policy_arn instead of requiring users to use property to manually define the AWS::IAM::User ManagedPolicyArns property.

templates/iam/integrations_users.rb:12:in `block (2 levels) in <module:IAM>': undefined method `managed_policy_arn' for #<Convection::Model::Template::Resource::IAMUser:0x007fae4befc0d0> (NoMethodError)
        from /Users/ecarey/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/convection-0.2.31/lib/convection/model/template.rb:22:in `instance_exec'
        from /Users/ecarey/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/convection-0.2.31/lib/convection/model/template.rb:22:in `block in attach_resource'
        from /Users/ecarey/rapid7/platform-delivery/.../templates/iam/integrations_users.rb:8:in `block in <module:IAM>'