allow replacing a node in an existing cluster

Magnum could allow the replacement of a given node explicitly. This would be useful in cases the user detects a misbehaving node with no obvious fix, and just decides to replace it with a brand new one.

This is possible today using heat commands, as in:

heat stack-update -x -P minions_to_remove=<ip-of-node-to-remove> <stack-id>

Heat will delete that node and launch a new one to keep the expected node_count. The only drawback is that this change is only done at the stack level, not propagated to magnum which will still show the old list of node_addresses.

One option is to add a new param in 'magnum cluster-update', something like:

magnum cluster-update <cluster-id> replace node=<node-ip>

which internally will trigger the stack-update using minions_to_remove as above. This parameter only seems to exist in kubernetes drivers, we would need to do something similar for the others.