ACCUMULO-4605 Remove hard-coded commands from Main

Add usageGroup to KeywordExecutable, so the Main class can print usage
of commands in categorical groups.
diff --git a/src/main/java/org/apache/accumulo/proxy/Proxy.java b/src/main/java/org/apache/accumulo/proxy/Proxy.java
index ee6067d..438e0d3 100644
--- a/src/main/java/org/apache/accumulo/proxy/Proxy.java
+++ b/src/main/java/org/apache/accumulo/proxy/Proxy.java
@@ -113,6 +113,11 @@
   }
 
   @Override
+  public UsageGroup usageGroup() {
+    return UsageGroup.PROCESS;
+  }
+
+  @Override
   public String description() {
     return "Starts Accumulo proxy";
   }