High memory usage of nova-api or possible crash due to simple tenant usage report within horizon

Issue

Frequent calls to Nova's API when displaying the simple tenant usage can lead to efficiency problems and even crash on the Nova side, especially when there are a lot of deleted nodes in the database. We are working on resolving that, but in the mean time, it would be nice to have a way of disabling the simple tenant usage stats on the Horizon side as a workaround.

Horizon enabled that option depending on whether it's supported on the Nova side. In the 2.0 version of API we can simply disable the support for it on the Nova side, but that won't be possible in version 2.1 anymore, so we need a configuration option on the Horizon side.