influxd upgrade writes config.toml to a confusing location

[danxmoran]

When influxd upgrade converts a V1 config file to V2, it writes the output config.toml into the same directory as the V1 file. This results in confusing behavior like config.toml being written into ~/.influxdb instead of ~/.influxdbv2. We don't expose an option to override this behavior.

We should fix the command to:

1. Expose a new CLI option to set the output path of the V2 config
2. Update the default path to be ~/.influxdbv2/config.toml