Support external_ip flag for gce_vm_image_export

[bpl4vv]

In a similar vein to #185, GCE Image Export Tool spins up a temporary VM while performing image export. This VM is often created with an external IP address.

This CL provides a flag so that the temporary VM can be created with a static external IP, an ephemeral IP, or no external IP.

[google-oss-prow]

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[google-oss-prow]

@bpl4vv: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
prow/presubmit/gocheck/cli-tools	628a448	link	true	/gocheck-cli-tools
prow/presubmit/gotest/cli-tools	628a448	link	true	/gotest-cli-tools
prow/presubmit/gobuild/cli-tools	628a448	link	true	/gobuild-cli-tools

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