Change monitor element schema to TypeString

[mnaboka]

The element type is set to schema.TypeInt, which does not allow us to
do interpolation. By changing this type to string all seem to work
correctly.

Our terraform configuration

monitor {
  id = "${module.poller_latency.id}"
}

The error we are seeing is this:

widget.0.monitor (id): cannot parse '' as int: strconv.ParseInt: parsing "${module.poller_latency.id}": invalid syntax

Feel free to close this PR if this is a not relevant / breaking change, i could be missing context here.

[bkabrda]

This PR seem to be correct. Testing locally without your PR, I can't even use a literal int as id, as the code in buildWidgets expects it to be a string and that causes Terraform to crash. I don't see how anyone could actually be using this, hence I don't see a possibility to break backwards compatibility. Using your PR makes everything work, so I'm merging it.

Thanks a lot for submitting!