indexoperations methods should take environment.Paths

[ahmetb]

While trying to refactor indexoperations in #549 and adding a DeleteIndex method I realized these methods should take environment.Paths value and use its method to construct paths to index dirs etc.

That's precisely why we created Paths and its methods: To avoid spread and duplication of FS layout of things and repetition of filepath.Joins all over the codebase. :)

/assign

I'll get to this once #549 is merged.
cc: @chriskim06