Fix nodes sharing mutable environment variable map

Patch by Ariel Weisberg; Reviewed by Joshua McKenszie for CASSANDRA-19551

ccmlib/node.py

@@ -148,7 +148,7 @@ def __init__(self,
         self.__install_dir = None
         self.__global_log_level = None
         self.__classes_log_level = {}
-        self.__environment_variables = environment_variables or {}
+        self.__environment_variables = environment_variables.copy() or {}
         self.__original_java_home = None
         self.__conf_updated = False