This PR updates the `pkg/testing/integration` package to support running integration tests against the Pulumi Service if desired. This is done through adding new options to `ProgramTestOptions`. (Generally adding support for providing values to flags that were previously inaccessible.)
I added an integration test to confirm that it all works if the PULUMI_API environment variable is set. These tests aren't run in Travis, only manually. Since we cannot reliably run tests from `master` against the service because of the delay in rolling out updates to the Pulumi SDK, etc.
