notifiers: Stop using url.Parse in validating address format. (#4011)

url.Parse() wrongly parses an address of format "address:port"
which is fixed in go1.8.  This inculcates a breaking change
on our end. We should fix this wrong usage everywhere so that
migrating to go1.8 eventually becomes smoother.

cmd/admin-handlers_test.go

@@ -1377,7 +1377,7 @@ func TestWriteSetConfigResponse(t *testing.T) {
 		},
 	}
 
-	testURL, err := url.Parse("dummy.com")
+	testURL, err := url.Parse("http://dummy.com")
 	if err != nil {
 		t.Fatalf("Failed to parse a place-holder url")
 	}