[DigitalOcean] add e2e tests

[srikiz]

Initial work for adding e2e tests for DO.

Tested locally and it seems to work fine after adding the below command arguments.
go run kubetest2-kops/main.go --up --cloud-provider digitalocean --cluster-name test.myclustername.com \ --env KOPS_STATE_STORE=do://***-kops-space \ --env DIGITALOCEAN_ACCESS_TOKEN=bd5f*****472dae3 \ --env S3_ENDPOINT=sfo2.digitaloceanspaces.com \ --env S3_ACCESS_KEY_ID=PPO******NSRW \ --env S3_SECRET_ACCESS_KEY=rm******oIc \ --kops-binary-path /home/kops \ --ssh-public-key ~/.ssh/id_rsa.pub \ --kubernetes-version 1.19.2

I'll separately work with @timoreimann and @rifelpet to include the necessary environment variables.

[k8s-ci-robot]

Hi @srikiz. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[rifelpet]

nice work! glad to see it isn't much effort here to add support
/ok-to-test

[rifelpet]

/retest
/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: rifelpet, srikiz

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[rifelpet]

/retest

[timoreimann]

Do we have a DO test account set up with kops already?

I know there's one that Cluster API uses which is good for sharing with other CNCF projects, but wasn't sure if that has happened already (or may be happening implicitly).