swift/ringbuilder: allow to disable the ring building

When using Hiera, it's usefull to have the "enabled" parameter to either
enable or disable the ring building, since it should be run on one
single node.

Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge pull request #508 from enovance/ringbuilder-enable

swift/ringbuilder: allow to disable the ring building