Extract metadata class

Since my next project requires me to make changes to the metadata drive, I decided to write unit tests for it.

I noticed that it's currently not modular at all and thus not testible. I extracted the metadata items creation into a modular class.

Unfortunately more work needs to be done before this is testible. (Need to be able to access both hva.conf and dcmgr.conf in tests) I have a mikado graph of further work to be done but this should already be mergable.

Since I started this work with the intention of writing tests, it's based on https://github.com/axsh/wakame-vdc/pull/395