Optionally check for diskimages on nodepool log generation tool

This tool assumes that diskimages option is always present on
nodepool configuration. For downstream consumption, it may not
be the case, so checking if setting is present.

Change-Id: I901e044ae9ec37eb64bc662e4182d317cf4a7647

tools/nodepool_log_config.py

@@ -129,8 +129,9 @@ def _get_providers_and_images(config_file):
     logging.debug("Added %d providers & images" % len(ret))
 
     # diskimages have a special provider
-    for diskimage in config['diskimages']:
-        ret.append(('dib', diskimage['name']))
+    if 'diskimages' in config:
+        for diskimage in config['diskimages']:
+            ret.append(('dib', diskimage['name']))
 
     return ret