Fix variable name for graphite basic auth.

It looks like in the refactoring of PR #144 the variable name for graphite basic auth got changed d79dcb952d5519b3abbb151c0802177cdcb31358

This fixes that so HTTP auth works again on graphite.

I would assume that a similar change would need to be made on elasticsearch, but I do not have a running instance to test against, so I don't feel comfortable making that change.

References #152.