Default track support in snap builds

[Adam Collard]

If a snap has a non-default default track (i.e. a default track has been specified, so that the default-track is not "latest") the snap build configuration page is confusing/misleading.

e.g. https://launchpad.net/~maas-committers/+snap/maas-master

This snap has a default-track of 2.7, and the form to configure it has a track field, with a field description of "Track defines a series for your software. If not specified, the default track ('latest') is assumed."

I want that snap to push explicitly to latest/edge, and not <default>/edge (note the behaviour now is correct).

Attempting to manually enter "latest" into the field results in it being returned back as empty (since, i assume, it matches the default).

lib.lp.snappy.browser.widgets.storechannels.StoreChannelsWidget has a _default_track = 'latest', which instead should be some sort of null object to allow the user to talk about either the default track implicitly, or "latest" explicitly.