Consider it successful to delete a non-existing FIP

[odaira]

Sometimes users need to destroy a resource without refresh terraform destroy -refresh=false for various reasons. Even when the resource does not exist any more, the delete operation should be considered successful.

This PR allows deletion of a non-existing FIP. It is already allowed for a security group rule:

terraform-provider-ibm/ibm/resource_ibm_is_security_group_rule.go

Lines 286 to 296 in b9fb8ae

	err = sgC.DeleteRule(secgrpID, ruleID)
	if err != nil {
	iserror, ok := err.(iserrors.RiaasError)
	if ok {
	if len(iserror.Payload.Errors) == 1 &&
	iserror.Payload.Errors[0].Code == "not_found" {
	return nil
	}
	}
	return err
	}

[odaira]

Let me know if you have any concern with allowing deletion of a non-existing resource without refresh.

An example case where users need to run terraform destroy -refresh=false: hashicorp/terraform#15386

[hkantare]

@odaira Intially I was concerned with the change because according to earlier docs of Terraform if its fails to delete any resource we should return back error..but yesterday I was going through latest Terraform docs
https://www.terraform.io/docs/extend/writing-custom-providers.html#implementing-destroy

The destroy function should always handle the case where the resource might already be destroyed (manually, for example). If the resource is already destroyed, this should not return an error. 

For Read also they are saying to follow the same logic
https://www.terraform.io/docs/extend/writing-custom-providers.html#implementing-read
can we update Read Method also?

[odaira]

@hkantare I've never thought about Read, but yes, looks like we need to update Read as well to gracefully handle already-deleted resources.

However, I've used Terraform with IBM plugin for already-deleted resources many times, but unless I specified -refresh=false, I've never seen any weird error. I guess even with the current plugin implementation, Terraform gracefully handles an error from Read and simply considers that the resource no longer exists. (This is just my guess.)