impossible to create a model assertion compatible with both snapd 2.13 and 2.14

[Steve Langasek]

In snapd 2.13, model assertions had 'core' (and others) as a mandatory field.

In snapd 2.14.2, including the 'core' field (and others) causes the assertion to be treated as invalid.

As long as snapd 2.14.2 has not made it into yakkety from yakkety-proposed (due to failing autopkgtests), this makes it very difficult to do sane CI of ubuntu-image:

 - if we test against xenial, we must change our model assertion test cases to be compatible with 2.14.2. But xenial doesn't have the preferred version of e2fsprogs; tests then require root for loop mounts, and don't test compatibility with the e2fsprogs features we use in the ubuntu-image snap.
 - if we test against yakkety (which is what we currently do), then we are driving wrong version of snap prepare-image with wrong model assertion test data.

It would have been nice to not go from "mandatory" to "illegal" in a single revision of snapd. Please either allow these fields in the model assertion on a transitional basis, or give priority to the autopkgtest failures of snapd 2.14.2 in yakkety-proposed so that ubuntu-image isn't left in limbo.

[Samuele Pedroni (pedronis)]

the transition happened as described in a backward incompatible manner but at this now much later point we don't plan to redo/undo this.