Add cache list support

[kairen]

This commit added cache list support. Because currently cannot know which images are cached.

For command result as below:

$ ./out/minikube cache add ubuntu:16.04 redis:3
$ ./out/minikube cache list
redis:3
ubuntu:16.04

[minikube-bot]

Can one of the admins verify this patch?

[dlorenc]

@minikube-bot test this please

[dlorenc]

@minikube-bot ok to test

[kairen]

Hi, @r2d4 Do you mind reviewing this PR?

[dlorenc]

Could you move this up out of the for loop? It looks like the format here is always a constant, so there's no need to do it in each iteration of the loop.

[kairen]

I added a new flag to custom format for cache list, for future work, we could extend the CacheListTemplate to add more image detail, such as below:

var cacheListFormat  = "{{.CacheImageName}}:{{.CacheImageTag}}\n"

type CacheListTemplate struct {
	CacheImageName   string
        CacheImageTag    string
}

But we need to ensure minikube cache add <image> use latest tag to save in config.

This <Image> does not have a tag.

For example:

$ minikube cache add redis
$ cat .minikube/config/config.json
{
    "WantReportError": true,
    "WantReportErrorPrompt": false,
    "cache": {
        "redis:latest": null
    },
    "ingress": false
}

[dlorenc]

Nit: do we need the "-" character here? It would make automatic parsing of stdout a little tricker (like if you wanted to use this in a bash for loop, etc.).

[kairen]

Yes, I ignored this usage scenario, I'll remove this character to bring about this results:

$ minikube cache add redis ubuntu
$ minikube cache delete $(minikube cache list)

[dlorenc]

Would a "slice" of images work instead of a map? It looks like you only set "nil" as the value.

[kairen]

No problem, I'll do it!