docs(troubleshooting): add a new item into troubleshooting

[DjangoPeng]

Provide a solution for Unknown variable: env issue.

Ref: #598

/assign @jlewi
/assign @gaocegege

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This change is 

[jlewi]

Can you include the error message that you saw in the bug?

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[DjangoPeng]

comment addressed @jlewi

[DjangoPeng]

No idea about the kubeflow-presubmit. It seems like all 7 tests succeeded, but e2e minikube failed.

@jlewi PTAL

[DjangoPeng]

@gaocegege PTAL

[jlewi]

/test all

[googlebot]

So there's good news and bad news.

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[jlewi]

@DjangoPeng I made some suggested changes to the wording of the error message PTAL.

[jlewi]

@DjangoPeng Do my changes look good to you? It would be good to get this merged and unblock your other PR #604 adding the TOC?

[DjangoPeng]

@jlewi LGTM

Sorry for the late reply. All day outside, just come back now.

[DjangoPeng]

/retest

[DjangoPeng]

From the testing error log, I guess there are some trouble in our CI env?

Command: go run hack/e2e.go -v --test --test_args='--ginkgo.focus=simple\-tfjob\-gke$'

Timeout waiting for simple-tfjob-gke in namespace kubeflow-presubmit-kubeflow-e2e-gke-602-28b03b9-1115-c7f3 to finish.

[jlewi]

Doesn't look like a quota issue

gcloud --project=kubeflow-ci compute regions list
NAME                     CPUS    DISKS_GB    ADDRESSES  RESERVED_ADDRESSES  STATUS  TURNDOWN_DATE
asia-east1               0/24    0/4096      0/8        0/8                 UP
asia-northeast1          0/24    0/4096      0/8        0/8                 UP
asia-south1              0/24    0/4096      0/8        0/8                 UP
asia-southeast1          0/24    0/4096      0/8        0/8                 UP
australia-southeast1     0/24    0/4096      0/8        0/8                 UP
europe-west1             0/24    0/4096      0/8        0/8                 UP
europe-west2             0/24    0/4096      0/8        0/8                 UP
europe-west3             0/24    0/4096      0/8        0/8                 UP
europe-west4             0/24    0/4096      0/8        0/8                 UP
northamerica-northeast1  0/24    0/4096      0/8        0/8                 UP
southamerica-east1       0/24    0/4096      0/8        0/8                 UP
us-central1              0/320   0/4096      0/32       0/8                 UP
us-east1                 72/320  6200/16384  9/32       0/8                 UP
us-east4                 0/24    0/4096      0/8        0/8                 UP
us-west1                 0/24    0/4096      0/8        0/8                 UP

[jlewi]

/test all

[jlewi]

/test all

[jlewi]

/test all

[jlewi]

Woo Hoo tests passed.

Merging this manually; because the CLA check is busted.