fix issue that e2e script exits due to unbound variables

[Huang-Wei]

What this PR does / why we need it:

In a kubeadm env, when running go run hack/e2e.go -- --provider=skeleton --test --test_args="--ginkgo.focus=TaintBasedEvictions", it outputs:

2018/10/12 19:18:42 e2e.go:79: Calling kubetest --verbose-commands=true --provider=skeleton --test --test_args=--ginkgo.focus=TaintBasedEvictions...
2018/10/12 19:18:42 process.go:153: Running: ./hack/e2e-internal/e2e-status.sh
Skeleton Provider: prepare-e2e not implemented
......
/root/go/src/k8s.io/kubernetes/cluster/../cluster/skeleton/util.sh: line 22: KUBE_MASTER_IP: unbound variable
2018/10/12 19:23:08 process.go:155: Step './hack/ginkgo-e2e.sh --ginkgo.focus=TaintBasedEvictions' finished in 30.036621ms
2018/10/12 19:23:08 main.go:311: Something went wrong: encountered 1 errors: [error during ./hack/ginkgo-e2e.sh --ginkgo.focus=TaintBasedEvictions: exit status 1]
2018/10/12 19:23:08 e2e.go:81: err: exit status 1
exit status 1

Special notes for your reviewer:

I'm using latest master branch in k/k.

Release note:

NONE

/sig testing

[Huang-Wei]

/hold
as I should be able to provide more info after I finish my e2e test.

[neolit123]

/kind bug

[Huang-Wei]

/hold cancel

@BenTheElder PTAL.

[BenTheElder]

not sure if this is actually correct, will leave to cluster/ reviewers

[roberthbailey]

This just allows the method to not fail if those env vars aren't set, which seems fine in the skeleton implementation.

/lgtm

[roberthbailey]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Huang-Wei, roberthbailey

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