New flag --repo-prefix to backfill --default-repo functionality

### Expected behavior

Previously I had been using `--default-repo` more like `--repo-prefix` this all came to a grinding halt when I attempted to use the below `skaffold.yaml`. When I ran it with

> skaffold build --default-repo=abc.123.com

I expected to see:

```
building abc.123.com/bases/go/1.13

building abc.123.com/bases/go/1.14
```

### Actual behavior

```
building abc.123.com/bases_go_1.13

building abc.123.com/bases_go_1.14
```
### Information

- Skaffold version: v1.10.1
- Operating system: Linux
- Contents of skaffold.yaml:

```yaml
apiVersion: skaffold/v1beta15
kind: Config

build:
artifacts:
- image: bases/go/1.13
context: bases/go/multi
docker:
buildArgs:
GO_VERSION: "1.13"
ALPINE_VERSION: "3.12"
- image: bases/go/1.14
context: bases/go/multi
docker:
buildArgs:
GO_VERSION: "1.14"
ALPINE_VERSION: "3.12"
```
### Proposal

We introduce a new flag `--repo-prefix` which doesn't mangle the image names for Docker Hub and simply applies the prefix.