There should be a way to force a node to leave the cluster

There are a few cases when we need to force a node to leave the cluster, see [this thread](https://groups.google.com/forum/?utm_medium=email&utm_source=footer#!msg/influxdb/nXI8ZawT1JQ/FeBRBBFAeOcJ) for an example. We need to find a way to force a node to leave the cluster in case there's no quorum.