Nest config validation (#1810)

* Config validation for Nest platforms.

homeassistant/components/nest.py

@@ -4,6 +4,9 @@ Support for Nest thermostats.
 For more details about this platform, please refer to the documentation at
 https://home-assistant.io/components/thermostat.nest/
 """
+import logging
+import socket
+
 import voluptuous as vol
 
 from homeassistant.const import CONF_PASSWORD, CONF_USERNAME
@@ -20,14 +23,27 @@ CONFIG_SCHEMA = vol.Schema({
     })
 }, extra=vol.ALLOW_EXTRA)
 
+_LOGGER = logging.getLogger(__name__)
+
+
+def devices():
+    """Generator returning list of devices and their location."""
+    try:
+        for structure in NEST.structures:
+            for device in structure.devices:
+                yield (structure, device)
+    except socket.error:
+        _LOGGER.error("Connection error logging into the nest web service.")
+
 
 # pylint: disable=unused-argument
 def setup(hass, config):
     """Setup the Nest thermostat component."""
     global NEST
 
-    username = config[DOMAIN].get(CONF_USERNAME)
-    password = config[DOMAIN].get(CONF_PASSWORD)
+    conf = config[DOMAIN]
+    username = conf[CONF_USERNAME]
+    password = conf[CONF_PASSWORD]
 
     import nest