Fix test_migrate_disk_and_power_off_exception
Fix test_migrate_disk_and_power_off_exception

The test asserts that Exception is raised, which means any number of
failures within the test itself will cause it to pass. In this case
there were two bugs in the test case:

1) Too many arguments to migrate_disk_and_power_off
2) fake_get_instance_disk_info should return a JSON string

I changed the assert to catch an AssertionError exception instead to
avoid this kind of bug in the future

Change-Id: I5e0f57b093ee1b2830696542d343c2dd89c383d6

n0ano/ganttclient