Make code more compact for get_list_opts function

Do it like in get_show_opts done.

Change-Id: If6b115c35a41997627660830fb8e5d299e8fb8e9

Author: kromanenko

functional/common/test.py

@@ -63,9 +63,7 @@ def get_show_opts(cls, fields=[]):
 
     @classmethod
     def get_list_opts(cls, headers=[]):
-        opts = ' -f csv '
-        opts = opts + ' '.join(['-c ' + it for it in headers])
-        return opts
+        return ' -f csv ' + ' '.join(['-c ' + it for it in headers])
 
     @classmethod
     def assertOutput(cls, expected, actual):