Fix load() missing argument Loader

This patch replace yaml.load() by yaml.safe_load() which will
fix the TypeError our jobs currently face:
TypeError: load() missing 1 required positional argument: 'Loader'

Change-Id: I3d238eabf374be895782ebc9e4907db93157a5d4

refstack_client/refstack_client.py

@@ -66,7 +66,7 @@ def get_input():
 def read_accounts_yaml(path):
     """Reads a set of accounts from the specified file"""
     with open(path, 'r') as yaml_file:
-        accounts = yaml.load(yaml_file)
+        accounts = yaml.safe_load(yaml_file)
     return accounts