Fix failing AWS and DigitalOcean tests

[cnunciato]

This PR fixes a few failing tests:

1. For aws-py-ec2-provisioners, tweaks the test assertion slightly to account for a change in the behavior of Pulumi Command. Until recently, the provider'sstdout property was an output of string, but it's now marked as a secret (see pulumi/command#256), and I wasn't able to get the test framework to unwrap the secret. (Anton's comment on the linked issue seems to suggest that may not even possible.) So I've adjusted the test to check for a non-empty value instead, which to me seems fine for now. (The program itself is unaffected.) I also went ahead and updated the program to use the latest version of Pulumi Command as well.

2. For digitalocean-{py,cs}-loadbalanced-droplets, updates the image ID used to provision DigitalOcean droplets. Apparently Ubuntu 18 is no longer valid.

Fixes #1494.
Fixes #1484.
Fixes #1491.

[pulumi-staging]

🍹 The Update for moolumi/k8s-ci-cluster/4b5971a00e4c2ae809eb170b787921e2499195e8-1518 was successful.

Resource Changes

    Name                                                          Type                                        Operation
+   gke                                                           pulumi:providers:kubernetes                 create
+   k8s-ci-cluster-4b5971a00e4c2ae809eb170b787921e2499195e8-1518  pulumi:pulumi:Stack                         create
+   multicloud                                                    pulumi-kubernetes:ci:GkeCluster             create
+   password                                                      random:index/randomPassword:RandomPassword  create
+   ephemeral-ci-cluster                                          gcp:container/cluster:Cluster               create
+   primary-node-pool                                             gcp:container/nodePool:NodePool             create

[pulumi-staging]

🍹 The Destroy for moolumi/k8s-ci-cluster/4b5971a00e4c2ae809eb170b787921e2499195e8-1518 was successful.

Resource Changes

    Name                  Type                                        Operation
-   gke                   pulumi:providers:kubernetes                 delete
-   primary-node-pool     gcp:container/nodePool:NodePool             delete
-   ephemeral-ci-cluster  gcp:container/cluster:Cluster               delete
-   password              random:index/randomPassword:RandomPassword  delete

[scottslowe]

Approved, but how does this relate to #1492 and other associated issues?

[scottslowe]

Also appears to be related to #1491

[cnunciato]

Thanks for the review @scottslowe!

• I think we can close Pin pulumi-command in aws-py-ec2-provisioners #1492 as you note, yeah, since there's technically nothing wrong with the program itself; it's just the test began failing because of the output type change.
• I don't think this is related to Update __main__.py #1493, but let me know if I'm overlooking something there.
• It fixes TestAccAwsPyEc2Provisioners test failing #1491 (since that issue is about the failing test, and a follow up's been filed on pulumi/command).