bin/cluster supports boto credentials as well as env variables

bin/cluster

@@ -143,8 +143,14 @@ class Cluster(object):
 
             inventory = '-i inventory/aws/hosts'
 
-            missing = [key for key in ['AWS_ACCESS_KEY_ID', 'AWS_SECRET_ACCESS_KEY'] if key not in os.environ]
-            if len(missing) > 0:
+            key_vars = ['AWS_ACCESS_KEY_ID', 'AWS_SECRET_ACCESS_KEY']
+            key_missing = [key for key in key_vars if key not in os.environ]
+
+            boto_conf_files = ['~/.aws/credentials', '~/.boto']
+            conf_exists = lambda conf: os.path.isfile(os.path.expanduser(conf))
+            boto_configs = [ conf for conf in boto_conf_files if conf_exists(conf)]
+
+            if len(key_missing) > 0 and len(boto_configs) == 0:
                 raise ValueError("PROVIDER aws requires {} environment variable(s). See README_AWS.md".format(missing))
 
         elif 'libvirt' == provider: