Add support for additional cloudwatch options

[Fitzsimmons]

• custom_metrics_namespaces and assume_role_arn now supported
• changes vendored version of go-grafana-api until upstream merges the
  relevant pull request (Add Namespaces of Custom Metrics nytm/go-grafana-api#7)

This should satisfy the requirements of #14.

[pearkes]

This LGTM. After merging the changes in #17 I get a clean test run:

$ 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 (0.72s)
=== RUN   TestAccDashboard_basic
--- PASS: TestAccDashboard_basic (0.44s)
=== RUN   TestAccDashboard_disappear
--- PASS: TestAccDashboard_disappear (0.37s)
=== RUN   TestAccDataSource_basic
--- PASS: TestAccDataSource_basic (0.67s)
PASS
ok  	github.com/terraform-providers/terraform-provider-grafana/grafana	2.214s

[pearkes]

Actually it may also be good to add a test case for this as well.

[pearkes]

Thanks @Fitzsimmons I rebased and merged this to master, added tests specific to cloudwatch. hashicorp/terraform-provider-grafana@8f85117...master