Update defaults and add tests for them

test/valid-config-default-values.hcl

@@ -1,6 +1,11 @@
 check_interval = "1s"
 default_alert_down = ["log_command"]
-default_alert_after = 1
+default_alert_every = 0
+default_alert_after = 2
+
+monitor "Default" {
+  command = ["echo"]
+}
 
 monitor "Command" {
   command = ["echo", "$PATH"]