always offer to pull latest image when creating a toolbox

**Is your feature request related to a problem? Please describe.**
We frequently see bug reports caused by people having an old toolbox image cached locally.

**Describe the solution you'd like**
`toolbox create` should offer to pull down the latest image being requested.
For extra points it could check if a newer image exists first.

**Additional context**
I am imagining something like this:
```
$ toolbox create
A newer fedora-toolbox:34 image is available, do you want to download it? [Y/n]
```

I think this is the least surprising behavior (though personally I would always want to pull the latest image).