Walking a tightrope between security and convenience here, because
git-annex-shell needs to only proxy for things when there has been
an explicit, local action to configure them.
In this case, the user has to have run `git-annex extendcluster`,
which now sets annex-cluster-gateway on the remote.
Note that any repositories that the gateway is recorded to
proxy for will be proxied onward. This is not limited to cluster nodes,
because checking the node log would not add any security; someone could
add any uuid to it. The gateway of course then does its own
checking to determine if it will allow proxying for the remote.
When there are multiple gateways to a cluster, this sets up proxying
for nodes that are accessed via a remote gateway.
Eg, when running in nyc and amsterdam is the remote gateway,
and it has node1 and node2, this sets up proxying for
amsterdam-node1 and amsterdam-node2. A client that has nyc as a remote
will see proxied remotes nyc-amsterdam-node1 and nyc-amsterdam-node2.
Just look at the existing proxied remotes that correspond to already
existing nodes of the cluster, and keep those nodes in the cluster.
While adding any remotes of the local repo that are configured as
cluster nodes. This allows removing cluster nodes from the local repo
and updating, without it also removing nodes provided by other gateways.
One benefit of this is that a typo in annex-cluster-node config won't
init a new cluster.
Also it gets the cluster description set and is consistent with
initremote.
