dovetail tool: add debug option to control the log level showing in screen

dovetail tool: add debug option to control the log level showing in screen

1. add debug option in cmd_config.yml for controling log showing in screen.
2. rename existent DEBUG as CON_DEBUG because it is used for functest/yardstick
   container to control the log level.
3. fix the bug in function update_config_envs(). If one key is not in system's
   envs, os.environ[key] get an error with ErrorKey.

JIRA: DOVETAIL-148

Change-Id: If3d75109f51e2e91743b8f1134a76da42e57383a
Signed-off-by: xudan <xudan16@huawei.com>