Update changelog for 0.19.0

[lblackstone]

No description provided.

[lblackstone]

@hausdorff CI build keeps failing the Istio integration test with the following error:

[                          step1 ]  +  pulumi:pulumi:Stack istio-p-it-travis-job-step1-aab215dc created 
[                          step1 ]  
[                          step1 ] Diagnostics:
[                          step1 ]   gcp:container:Cluster (test-ci-istio):
[                          step1 ]     error: Plan apply failed: 1 error occurred:
[                          step1 ]     
[                          step1 ]     * creating urn:pulumi:p-it-travis-job-step1-aab215dc::istio::gcp:container/cluster:Cluster::test-ci-istio: Invalid address to set: []string{"cluster_autoscaling"}

[lukehoban]

Invalid address to set: []string{"cluster_autoscaling"}

That is pulumi/pulumi-gcp#79, which is being fixed in pulumi/pulumi-gcp#81.