t/lb_target_group: Add missing matcher in acc test

[radeksimko]

We no longer set default matcher b/c there's no matcher for network LBs.
The following test started failing after merging #2251

=== RUN   TestAccAWSAutoScalingGroup_ALB_TargetGroups_ELBCapacity
--- FAIL: TestAccAWSAutoScalingGroup_ALB_TargetGroups_ELBCapacity (154.65s)
    testing.go:503: Step 0 error: Error applying: 1 error(s) occurred:
        
        * aws_lb_target_group.test: 1 error(s) occurred:
        
        * aws_lb_target_group.test: Error creating LB Target Group: ValidationError: Health check matcher HTTP code cannot be empty
            status code: 400, request id: 0fe27fc2-ca91-11e7-beee-571c8f0de551

I would not treat this as a breaking change per say, because it only affects new target groups. Existing ones will already have matchers in the state, so the removal of Default won't affect them.

That said I think we should document this in the NOTES section of changelog, e.g.

• resource/aws_lb_target_group: We no longer provide defaults for health_check's path nor matcher in order to support network load balancers where these arguments aren't valid. Creating new ALB will therefore require you to specify these two arguments. Existing deployments are unaffected.

Test Results (after patch)

TF_ACC=1 go test ./aws -v -run=TestAccAWSAutoScalingGroup_ALB_TargetGroups_ELBCapacity -timeout 120m
=== RUN   TestAccAWSAutoScalingGroup_ALB_TargetGroups_ELBCapacity
--- PASS: TestAccAWSAutoScalingGroup_ALB_TargetGroups_ELBCapacity (381.21s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	381.242s

[catsby]

👍

[ghost]

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