Support putting external images into the local registry of the kubevirtci cluster (#3801)

[gaohoward]

User can define an environment variable EXTRA_IMAGES that contains a list of external image urls separated by spaces.

At the end of 'make cluster-up' target the EXTRA_IMAGES are processed by a script ./cluster-up/extra-images.sh so that for each image url, it pulls the image and then push the image into the local registry.

Then when 'make cluster-sync' is invoked, it searches all external image urls in the generated yamls in _out/manifests/ and replaces them with corresponding urls in the local registry (registry:5000)

What this PR does / why we need it:

Added functionality for cluster-up target to allow adding external images to the internal local docker registry

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #3801

Special notes for your reviewer:

Release note:

Normal users should not affected by this feature. Only users with isolated environments can use this feature to add
any external images to the local registry. Make sure your local machine have access to the external repositories in order to pull them and push to the local registry.

[kubevirt-bot]

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[gaohoward]

@awels Hi Alex, just updated the branch. Pls take a look and see if it looks better. Thanks!