Fix incorrect variable names in README
Better handling of storage directories for carbon and graphite-web

We support now the two parameters `$graphite::carbon::storage_dir` and
`$graphite::web::storage_dir`, respectively.
Log directory of graphite-web is configurable via `$graphite::web::log_dir`
Log directory of carbon is configurable via `$graphite::carbon::log_dir`
Add input validation for newly added directory params