Update default data path

We want the data to land in the more clearly named /data directory.
Turn off call-home functionality

Bt default, InfluxDB calls home to report stats. This turns that off.
Change default backend to leveldb

We were having issues with InfluxDB crashing while using the default
RocksDB backend. Although RocksDB performs better than LevelDB, both
are very much performant enough for us and more imprtant is that it
doesn't crash.
General update of config file

This is just and update of the config file with some of the newer
options that are available.
Add DEFAULT_BACKEND environment variable

This allows us to change the backend via an environment variable.