Use keys specified in env vars for GCP and AWS

[dims]

Avoid creating new SSH keys if they are already specified. Some CI jobs are failing because the e2e tests are using what is in the env vars, but kops ends up creating new keys when these are not picked up.

Also log what we are doing so if we end up tripping up any CI jobs, we'll know quickly what went wrong.

[dims]

see below for example log of key generation from: https://prow.k8s.io/log?container=test&id=1829315846992826368&job=pull-kubernetes-e2e-gce-disruptive-canary

you can see DaemonRestart [Disruptive] are failing and the e2e tests are expecting the following keys to be used):

      - name: GCE_SSH_PRIVATE_KEY_FILE
        value: /etc/ssh-key-secret/ssh-private
      - name: GCE_SSH_PUBLIC_KEY_FILE
        value: /etc/ssh-key-secret/ssh-public

I0830 00:36:47.041297   15188 common.go:145] Using cluster name: e2e-pr127005.pull-kubernetes-e2e-gce-disruptive-canary.k8s.loca
I0830 00:36:47.058209   15188 ssh.go:43] gcloud compute --project=k8s-infra-e2e-boskos-063 config-ssh --ssh-key-file=/tmp/kops-ssh3991035025/key
WARNING: The private SSH key file for gcloud does not exist.
WARNING: The public SSH key file for gcloud does not exist.
WARNING: You do not have an SSH key for gcloud.
WARNING: SSH keygen will be executed to generate a key.
Generating public/private rsa key pair.
Your identification has been saved in /tmp/kops-ssh3991035025/key
Your public key has been saved in /tmp/kops-ssh3991035025/key.pub
The key fingerprint is:
SHA256:mPHOg54889XATlT7PUlJlt1atqCcdyJUahtatM5fkyI root@f86513e2-a6b9-47f2-8a16-0231e1f69a64
The key's randomart image is:
+---[RSA 3072]----+
|           .o. o+|
|          .oooo.*|
|      .   +*+ .*.|
|       = o*=o+o+o|
|      o S.+Eo.+*.|
|       + o oo o o|
|      . + o ..   |
|     oo. o       |
|      ++.        |

[hakman]

Pretty cool, thanks @dims! 😄

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