Use ForceDeleteMonitor instead of DeleteMonitor

[Towerthousand]

This implements #439

[platinummonkey]

I think I would prefer this to be somehow configurable. This is there to protect accidental deletes of resources (monitors in this case) that might be referenced in: dashboard widgets and SLOs for example.

[nmuesch]

@Towerthousand Thanks for this PR! To @platinummonkey's point, I think it would be helpful to maybe add an element to the schema so that users would explicitly be choosing to force delete their monitors and not have any accidental deletions?

I imagine the workflow in this case being, a user tries to delete a monitor without the force field set, and if its used somewhere they'd get a failure message with the monitor not deleted. Then, if it's really intended to delete, the force flag would be added to the schema with a terraform apply to store it in the state. Then remove the monitor and run another apply.
Though I'm not sure if this field can be read from the state from within the delete call?

[Towerthousand]

fixed in #535