How to fix cloud-config not work with error "Failed loading yaml blob"

If cloud-config on an AWS EC2 instance or OpenStack does not behave as configured, and the error log shows “Failed at merging in cloud config part from …” as below:

2014-05-10 12:14:12,458 - util.py[WARNING]: Failed loading yaml blob

2014-05-10 12:14:12,480 - util.py[WARNING]: Failed loading yaml blob

2014-05-10 12:14:12,481 - util.py[WARNING]: Failed at merging in cloud config part from part-001

Check that the YAML is valid. The problem may be tabs used instead of spaces. Validate it at http://www.yamllint.com/ and try again.