ovn: Improve clustering

Currently clustering steps are very static, if for a reason first
node in the inventory fails and gets re-introduced - K-A will create
a second empty cluster on that node.

This patch changes the approach and checks if cluster exists, if it
does - chooses a donor for the new node from currently running
node set.

Also it fixes node replacement - it removes old node from cluster
(that has the same ip address as newly provisioned node).

Closes-Bug: #1875223

Change-Id: Ia025283e38ea7c3bd37c7a70d03f6b46c68f4456