declare aws output as map

Change-Id: Iad85307d3a525cbf0e91dd373043632762f51047
diff --git a/src/com/mirantis/mk/Aws.groovy b/src/com/mirantis/mk/Aws.groovy
index 6b77171..d0ec4c4 100644
--- a/src/com/mirantis/mk/Aws.groovy
+++ b/src/com/mirantis/mk/Aws.groovy
@@ -132,7 +132,7 @@
 def getOutputs(venv_path, env_vars, stack_name, key = '') {
     def aws = new com.mirantis.mk.Aws()
     def common = new com.mirantis.mk.Common()
-    def output = {}
+    def output = [:]
 
     def stack_info = aws.describeStack(venv_path, env_vars, stack_name)
     common.prettyPrint(stack_info)