data_source: make URL field optional

[pearkes]

The CloudWatch datasource does not require the URL attribute via the API. This fixes #13.

With the changes cherry-picked from #17 tests pass.

$ make testacc
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v  -timeout 120m
?   	github.com/terraform-providers/terraform-provider-grafana	[no test files]
=== RUN   TestProvider
--- PASS: TestProvider (0.00s)
=== RUN   TestProvider_impl
--- PASS: TestProvider_impl (0.00s)
=== RUN   TestAccAlertNotification_basic
--- PASS: TestAccAlertNotification_basic (0.62s)
=== RUN   TestAccDashboard_basic
--- PASS: TestAccDashboard_basic (0.42s)
=== RUN   TestAccDashboard_disappear
--- PASS: TestAccDashboard_disappear (0.36s)
=== RUN   TestAccDataSource_basic
--- PASS: TestAccDataSource_basic (0.74s)
PASS
ok  	github.com/terraform-providers/terraform-provider-grafana/grafana	(cached)