Comment 1 for bug 1612313

This is fixed in d9537aaa37f1e17db334c7cf8888ea3c4dcf1436

cloud-init will now support MAAS having a 'vendor-data' key under meta-data/.
If it is present and non-empty, then it must be yaml loadable string (recognize that json is valid yaml, so sending json is valid).

The object represented by the yaml should be a dictionary with a 'cloud-init' key.

Example valid data to affect the 'packages' setting, would be:

{
 "cloud-init": "#cloud-config\npackages=['pastebinit']\n",
 "this-key-is-ignored": "something here"
}

Note that the 'cloud-init' is a *string*. It is not a dictionary.
It can also be a list, in which case each of the pieces will be consumed as a cloud-init part (much as it would read multipart-mime input).