add snapcraft.yaml & --extra-snaps

[mvo5]

This adds a snapcraft.yaml file for ubuntu-image and fixes a bug where mcopy can not be found in the snap because the PATH is reset.

Note that the snap is not fully working because ubuntu-image when called without sudo. In this case the code tries to call sudo itself which will not work (yet) inside snaps because we have not the right pty available for the asking for a password.

Of couse you need to adjust the git path once its merged, I had to have this to be able to test the image.

[mvo5]

Please also note that with current master my amd64 image does not boot with just kvm test.img.

[mvo5]

Sorry for the messy branch, things are in a bit of a hurry here. Happy to split this out into meaningful branches.

[mvo5]

The os.trunacte() one is super important, its the difference between a 4gb file and a much smaller file with a hole in it. This is important for our automatic image testing as the file with the hole can be fitted into RAM on our test instances but the 4gb real file can't.

[warsaw]

@mvo5 In what cases is $PATH reset? Do you have any additional details on why $PATH won't propagate to the child in the subprocess call?

[warsaw]

Is the problem that snap prepare-image should be calling unsquashfs using the full path, i.e. to /usr/bin/unsquashfs?

[mvo5]

@warsaw Thanks for your work on this! I followed up in the #46 PR and will close this one.