Pull command image name parsing and registry config support

[bburky]

The pull command appears to be unable to pull from docker.io when it gets a 401 UNAUTHORIZED:

$ docker run  gcr.io/uber-container-tools/makisu:latest pull --cacerts=/makisu-internal/certs/ busybox
{"level":"info","ts":1610039682.90314,"msg":"* Started pulling image index.docker.io/busybox:latest"}
panic: pull manifest: http send error: GET https://index.docker.io/v2/busybox/manifests/latest 401: {"errors":[{"code":"UNAUTHORIZED","message":"authentication required","detail":[{"Type":"repository","Class":"","Name":"busybox","Action":"pull"}]}]}

goroutine 1 [running]:
github.com/uber/makisu/bin/makisu/cmd.(*pullCmd).Pull(0xc000382550, 0x7fff6b1bcf79, 0x7)
    /workspace/github.com/uber/makisu/bin/makisu/cmd/pull.go:68 +0x33d
github.com/uber/makisu/bin/makisu/cmd.getPullCmd.func2(0xc000468000, 0xc0003697a0, 0x1, 0x2)
    /workspace/github.com/uber/makisu/bin/makisu/cmd/pull.go:44 +0x4a
github.com/spf13/cobra.(*Command).execute(0xc000468000, 0xc000369700, 0x2, 0x2, 0xc000468000, 0xc000369700)
    /go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:766 +0x2ae
github.com/spf13/cobra.(*Command).ExecuteC(0xc0003b5680, 0xc000243f60, 0x1, 0x1)
    /go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:852 +0x2ec
github.com/spf13/cobra.(*Command).Execute(...)
    /go/pkg/mod/github.com/spf13/cobra@v0.0.3/command.go:800
github.com/uber/makisu/bin/makisu/cmd.Execute()
    /workspace/github.com/uber/makisu/bin/makisu/cmd/root.go:79 +0x15a
main.main()
    /workspace/github.com/uber/makisu/bin/makisu/main.go:22 +0x20

This commit changes it's pull behavior to be more like FromStep by removing the explicit DefaultDockerHubConfiguration configuration.

The --registry and --tag arguments are removed, instead the image name is parsed with ParseNameForPull. This is a breaking change. It would be possible to change this to preserve backwards compatibility still use --registry and --tag if present.

The pull command did not allow providing a registry config. This PR adds a --registry-config argument in the same way as the build command.

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