Don't die in case of faulty node

This patch fixes TOCTOU issue introduced in the following commit:

* 93b9e37

If the node was just removed from the cluster, then there is a small
window when it is still listed as a member of a Mnesia cluster
locally. We retrieve list of nodes by calling locally

```erlang
 unsafe_rpc(Node, rabbit_mnesia, cluster_nodes, [running]).
```

However retrieving status from that particular failed node is no longer
available and throws an exception. See `alarms_by_node(Name)` function,
which is simply calls `unsafe_rpc(Name, rabbit, status, [])` for this
node.

This `unsafe_rpc/4` function is basically a wrapper over `rabbit_misc:rpc_call/4`
which translates `{badrpc, nodedown}` into exception. This exception
generated by `alarms_by_node(Name)` function call emerges on a very high
level, so rabbitmqct thinks that the entire cluster is down, while
generating a very bizarre message:

	Cluster status of node 'rabbit@overcloud-controller-0' ...
	Error: unable to connect to node 'rabbit@overcloud-controller-0':
	nodedown

	DIAGNOSTICS
	===========

	attempted to contact: ['rabbit@overcloud-controller-0']

	rabbit@overcloud-controller-0:
	  * connected to epmd (port 4369) on overcloud-controller-0
	  * node rabbit@overcloud-controller-0 up, 'rabbit' application running

	current node details:
	- node name: 'rabbitmq-cli-31@overcloud-controller-0'
	- home dir: /var/lib/rabbitmq
	- cookie hash: PB31uPq3vzeQeZ+MHv+wgg==

See - it reports that it failed to connect to node
'rabbit@overcloud-controller-0' (because it catches an exception from
`alarms_by_node(Name)`), but attempt to connect to this node was
successful ('rabbit' application running).

In order to fix that we should not throw exception during consequent
calls (`[alarms_by_node(Name) || Name <- nodes_in_cluster(Node)]`), only
during the first one (`unsafe_rpc(Node, rabbit_mnesia, status, [])`).

Even more - we don't need to change `nodes_in_cluster(Node)`, because it
is called locally. The only function which must use
`rabbit_misc:rpc_call/4` is `alarms_by_node(Name)` because it is
executed remotely.

See this issue for further details and real world example:

* https://bugzilla.redhat.com/1356169

Signed-off-by: Peter Lemenkov <[email protected]>

Author: lemenkov

src/rabbit_control_main.erl

@@ -953,6 +953,9 @@ nodes_in_cluster(Node) ->
     unsafe_rpc(Node, rabbit_mnesia, cluster_nodes, [running]).
 
 alarms_by_node(Name) ->
-    Status = unsafe_rpc(Name, rabbit, status, []),
-    {_, As} = lists:keyfind(alarms, 1, Status),
-    {Name, As}.
+    case rpc_call(Name, rabbit, status, []) of
+        {badrpc,nodedown} -> {Name, [nodedown]};
+        Status ->
+            {_, As} = lists:keyfind(alarms, 1, Status),
+            {Name, As}
+    end.