config: updated to deal with go-yaml sequence bug

I really wish there were a better way to do this. Basically, go-yaml has
a bug where it won't overwrite sequence values, it appends. I found this
when I configured an alternat cassandra host and it still tried to use
localhost.

Hopefully the go-yaml people fix it quickly enough (filed issue:
https://github.com/go-yaml/yaml/issues/48). I looked into doing it
myself and it was taking too long to figure out.

I tried using https://github.com/imdario/mergo to do a merging thing so
we're less likely to make any mistakes but it just plain didn't work.

Having spent too much time, I'm doing it the
quick-and-throughly-commented way and moving on with life. Unless anyone
has better ideas.