Add an explanatory tooltip to the unauthorized node drain popover

ui/app/templates/components/drain-popover.hbs

@@ -2,7 +2,12 @@
   data-test-drain-popover
   isDisabled=isDisabled
   label=(if client.isDraining "Update Drain" "Drain")
-  triggerClass=(concat "is-small " (if drain.isRunning "is-loading")) as |m|}}
+  tooltip=(if isDisabled "Not allowed to drain clients")
+  triggerClass=(concat
+    "is-small "
+    (if drain.isRunning "is-loading ")
+    (if isDisabled "tooltip is-right-aligned")
+  ) as |m|}}
   <form data-test-drain-popover-form onsubmit={{action (queue (action preventDefault) (perform drain m.actions.close))}} class="form is-small">
     <h4 class="group-heading">Drain Options</h4>
     <div class="field">

ui/app/templates/components/popover-menu.hbs

@@ -6,7 +6,8 @@
 >
   <dd.Trigger
     data-test-popover-trigger
-    class={{concat "popover-trigger button is-primary " triggerClass (if isDisabled "is-disabled")}}
+    class={{concat "popover-trigger button is-primary " triggerClass (if isDisabled " is-disabled")}}
+    aria-label={{tooltip}}
     {{on "keydown" (action "openOnArrowDown" dd)}}
   >
     {{label}} {{x-icon "chevron-down" class="is-text"}}