[carbon] allow setting empty carbon root node

This adds ability to use empty carbon root node, so that carbon-id will be out root.
It also adds dot at the end of user specified root node if it's missing.

I kept ```required_argument``` flag, so from command line user needs to explicitly set empty string ```""``` (IMHO should be more typo resistant than allowing argument-less --carbon-root).

Also ```requests``` metric was still using hardcoded root.