Expose network tags for GCE builder

[anmazzotti]

Change description

This PR exposes network tags for the GCE builder. See reference for tags usage.

This can be consumed from user set variables, with a comma separated list, see reference.

For example:

{
  "tags": "packer,foo,bar"
}

This is a small quality of life change to facilitate network management.

• Is this change including a new Provider or a new OS? No

Related issues

• Fixes #

Additional context

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[k8s-ci-robot]

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[alexander-demicev]

/ok-to-test

[mboersma]

This is blocked on #1888.

[mboersma]

/retest

[mboersma]

/lgtm
/approve

Thanks for adding this! Sorry it took us a while to work out our CI issues.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mboersma

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