Fix unhandled promise rejection in create monitor

If a monitor was created with an argument that was not a key-value type, a
promise would be rejected outside of the main maxctrl function. Added a
test case that covers this and fixed a few other test coverage problems
that were present.

maxctrl/test/drain.js

@@ -15,5 +15,10 @@ describe("Draining servers", function() {
             .should.eventually.have.string("Maintenance")
     })
 
+    it('does not drain non-existent server', function() {
+        return doCommand('drain server not-a-server')
+            .should.be.rejected
+    })
+
     after(stopMaxScale)
 });