Reenable signature workflow extended test

[miminar]

The extended signature workflow test, disabled by #15807, used to fail with:

/tmp/openshift/build-rpm-release/tito/rpmbuild-originV11XYu/BUILD/origin-3.7.0/_output/local/go/src/github.com/openshift/origin/test/extended/registry/signature.go:112
Expected
    <string>: error verifying signature sha256:248398a9acf9b2f80aabebee81ae88d70768eadebccecd786a0e6e1d2756e4cf@67655a940b7d40e9f4d3067765b613e7 for image sha256:248398a9acf9b2f80aabebee81ae88d70768eadebccecd786a0e6e1d2756e4cf (verification status will be removed): failed to get image "sha256:248398a9acf9b2f80aabebee81ae88d70768eadebccecd786a0e6e1d2756e4cf" manifest: Get http://docker-registry.default.svc:5000/v2/: malformed HTTP response "\x15\x03\x01\x00\x02\x02"
to contain substring
    <string>: identity is now confirmed
/tmp/openshift/build-rpm-release/tito/rpmbuild-originV11XYu/BUILD/origin-3.7.0/_output/local/go/src/github.com/openshift/origin/test/extended/registry/signature.go:106

The registry is secured during the extended test run. It looks like the oadm verify-image-signature cannot talk to secured registry. This needs to be fixed before re-enabling this test.

[miminar]

@mfojtik FYI. Feel free to take this over when you enhance the verify-image-signature command with --insecure flag.

[openshift-bot]

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[openshift-bot]

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