Fix resource warnings when running tests

There are some open calls that are not protected using with.

Change-Id: I98a45c4df38c7a22282fa6abf911a1815fb6bbfa

nodepool/cmd/config_validator.py

@@ -79,7 +79,8 @@ class ConfigValidator:
         log.info("validating %s" % self.config_file)
 
         try:
-            config = yaml.safe_load(open(self.config_file))
+            with open(self.config_file) as f:
+                config = yaml.safe_load(f)
         except Exception:
             log.exception('YAML parsing failed')
             return 1

nodepool/config.py

@@ -229,7 +229,8 @@ def openConfig(path):
     # attempt to reload it.
     while True:
         try:
-            config = yaml.safe_load(open(path))
+            with open(path) as f:
+                config = yaml.safe_load(f)
             break
         except IOError as e:
             if e.errno == 2:

nodepool/tests/unit/test_driver_aws.py

@@ -55,7 +55,8 @@ class TestDriverAws(tests.DBTestCase):
 
         ec2_template = os.path.join(
             os.path.dirname(__file__), '..', 'fixtures', 'aws.yaml')
-        raw_config = yaml.safe_load(open(ec2_template))
+        with open(ec2_template) as f:
+            raw_config = yaml.safe_load(f)
         raw_config['zookeeper-servers'][0] = {
             'host': self.zookeeper_host,
             'port': self.zookeeper_port,