cmd/snap,image: initial support for reproducible images, ability to parse and use seed.manifests

[Meulengracht]

This PR adds support for providing an seed.manifest file generated by ubuntu-image. The goal is to move the generation of seed.manifest out of ubuntu-image and let snap prepare-image generate it instead. To allow for a quicker implementation I've split this into two parts, with this PR being the first.

The next step is to coordinate the move of generating the seed.manifest out of ubuntu-image and then enable prepare-image to call WriteSeedManifest.

OBS: It's important to note here, that currently the store does not support fully this functionality, if you try to use this functionality there is a good chance the store will reject and say that you are not allowed to retrieve specific revisions due to missing permissions for snaps. This is something that needs to be enabled in store first.

OBS2: Due to this the spread test is currently using the fake-store, which doesn't exactly handle the revision, but instead logs the intention, and this is something we check for in the spread test to ensure that the communication between prepare-image and store is correct.

Once the official store supports this, then we should probably rewrite the spread test to use the official store instead.

[pedronis]

OBS: It's important to note here, that currently the store does not support this functionality, if you try to use this functionality there is a good chance the store will reject and say that you are not allowed to retrieve specific revisions. This is something that needs to be enabled in store first.

to be clear I think the store supports this in some cases with the right permissions

[Meulengracht]

OBS: It's important to note here, that currently the store does not support this functionality, if you try to use this functionality there is a good chance the store will reject and say that you are not allowed to retrieve specific revisions. This is something that needs to be enabled in store first.

to be clear I think the store supports this in some cases with the right permissions

Ah thanks, I updated the comment to reflect this a bit better. I hope I clarified this

[pedronis]

@Meulengracht there is a conflict now