A previous commit introduced a feature to specify the directory the
agent uses for shell class execution via a cfg entry, agent_dir.
However this addition introduced an error when using shaker-spot:
oslo_config.cfg.NoSuchOptError: no such option agent_dir in group
[DEFAULT]
This commit fixes shaker-spot while still supporting the agent_dir
feature
Change-Id: I7cfb87ba406e177121e7914225857937a19502e4