provider/datadog: Add aggregator method to timeboard graph resource
Adds aggregator argument to the timeboard graph resource.
Ex:
```
request {
q = "top(avg:docker.cpu.system{*} by {container_name}, 10, 'mean', 'desc')"
aggregator = "sum"
}
```
```
$ make testacc TEST=./builtin/providers/datadog TESTARGS='-run=TestAccDatadogTimeboard_update'
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/01/13 17:52:50 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/datadog -v -run=TestAccDatadogTimeboard_update -timeout 120m
=== RUN TestAccDatadogTimeboard_update
--- PASS: TestAccDatadogTimeboard_update (82.60s)
PASS
ok github.com/hashicorp/terraform/builtin/providers/datadog 82.619s
```
Fixes: #11168