Add the influxdb_legacy flag
Add the influxdb_legacy flag

This change introduces the influxdb_legacy flag to the plugin's
configuration. This flag is hidden and defaults to true. Developers
will use it to introduce gradually the modifications needed to support
InfluxDB v0.9.

Change-Id: I3b93153c43dc013464519de656e26ed45b8653ee
Implements: blueprint upgrade-influxdb-grafana

stackforge/fuel-plugin-lma-collector,stackforge/fuel-plugin-lma-collector,stackforge/fuel-plugin-lma-collector,stackforge/fuel-plugin-lma-collector,stackforge/fuel-plugin-lma-collector