Add ability to set labels on image push

[old-ocean-creature]

Add ability to set labels on image push

[vmwclabot]

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[old-ocean-creature]

fixes https://github.com/vmware-tanzu/carvel-imgpkg/issues/153

[vmwclabot]

@old-ocean-creature, VMware has approved your signed contributor license agreement.

[phenixblue]

@joaopapereira I believe this can be closed now since #552 was merged

[joaopapereira]

Closing since the work was already done