fix: replace / with - in anonymous ephemeral volume names (manager-specific) (manager#1598)

[iwilltry42]

dirs: "/foo/bar": "ephemeral://" stores a volume with name /foo/bar, which is OK in runtime, but is actually used as a resource name in Manager, which cannot contain /, so we're replacing this here (instead of adapting the store strategy in Manager)

I'm not sure if we'll need more sophisticated replaces here to make it a DNS name 🤔

Discovered as part of #1598

Checklist

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[g-linville]

This looks fine to me, but how badly will this disrupt existing volumes with / in the name from the user's perspective?

[iwilltry42]

@g-linville this only affects ephemeral:// (no name given) auto-generated volume names, so the impact will probably be fairly low, especially since they apparently didn't work in Manager so far.