Fix conf.py for the ReadTheDocs site
Fix conf.py for the ReadTheDocs site

The RTD_NEW_THEME variable isn't supported anymore and the html_theme
variable should be set to 'default' to let RTD use its own theme.

Change-Id: I9a94bb8d72a28ce25a7248f39aa7bbd6fe24a39a

stackforge/fuel-plugin-influxdb-grafana,stackforge/fuel-plugin-influxdb-grafana,stackforge/fuel-plugin-influxdb-grafana,stackforge/fuel-plugin-influxdb-grafana