Implement monitor_id support for downtime

[cvbarros]

This PR is a shot at trying to implement the monitor_id field for downtime.
Since this is my first PR for terraform, please let me know if there are any hard mistakes, and I'll more than happy to work through them.

[nyanshak]

I'd suggest also changing the docs to mention this new (optional?) field: https://github.com/terraform-providers/terraform-provider-datadog/blob/master/website/docs/r/downtime.html.markdown.

Also, I think you should check for errors on Sets, similar to @yfronto's proposed change (#27):

if err := d.Set("title", timeboard.GetTitle()); err != nil {
	return err
}

Otherwise, LGTM :)

[cvbarros]

Hey @nyanshak, thanks for looking into this.
I fixed some problems with acceptance tests and implemented the changes you suggested.

[HontoNoRoger]

@nyanshak Could we please get this into the master?

[ghost]

@HontoNoRoger HashiCorp manages this repo. I think @tombuildsstuff might be the right contact to ping on this.

[nyanshak]

(I really wish I could get it merged into master but I don't have the right permissions. Sorry @HontoNoRoger)

[tombuildsstuff]

hey @cvbarros

Thanks for this PR - apologies for the delayed review here!

I've taken a look through and this LGTM - I've also run the test suite and the suite passes too 👍

Thanks!

[tombuildsstuff]

Tests pass:

[kikitux]

https://github.com/terraform-providers/terraform-provider-datadog/issues/25