Allow IPv4 addresses for SNTP servers (#340)

Fixes https://github.com/OttoWinter/esphomelib/issues/395

esphomeyaml/config_validation.py

@@ -458,9 +458,12 @@ def hostname(value):
 
 def domain(value):
     value = string(value)
-    if re.match(vol.DOMAIN_REGEX, value) is None:
-        raise vol.Invalid("Invalid domain: {}".format(value))
+    if re.match(vol.DOMAIN_REGEX, value) is not None:
         return value
+    try:
+        return str(ipv4(value))
+    except vol.Invalid:
+        raise vol.Invalid("Invalid domain: {}".format(value))
 
 
 def domain_name(value):

tests/test1.yaml

@@ -1022,7 +1022,7 @@ time:
   servers:
   - 0.pool.ntp.org
   - 1.pool.ntp.org
-  - 2.pool.ntp.org
+  - 192.168.178.1
   on_time:
     cron: '/30 0-30,30/5 * ? JAN-DEC MON,SAT-SUN,TUE-FRI'
     then: