Handle non-array parameters for "queue" setting in supervisor config

[PrinsFrank]

When working with supervisors that only handle one queue, it is apparently possible to supply a single string instead of an array of strings (#1298):

'environments' => [
    'production' => [
        'supervisor-1' => [
-            'queue' => [
-                'default',
-                'high-prio'
-            ]
+            'queue' => 'default'
        ],
    ],
],

I can't find in the documentation if this was ever mentioned as a feature, but apparently this broke with the introduction of displaying crashed supervisors.

Fixes #1298