I was running into the same issue, and your post helped me out.
Looks like a quoting issue. I've escaped some of the double-quotes and switched to single quotes inside the dictionary. Maybe try this:
test-module -m /usr/share/ansible/monitoring/my_maintenance.py -a "schedule_type=One-Time devices=\"{'hostName':'HYDLPT326'}\" start_time=2015-06-01T16:00:00+0000 end_time=2015-06-01T16:30:00+0000 timezone='Asia/Calcutta' client_id=client_2655"
I was missing the double-quotes around the dictionary.
Good luck!