snapd is not tested against Debian

[Zygmunt Krynicki]

Spread supports Debian but our test suite is not actively exercised against it. A quick rough look at this suggests that the following steps must be taken:

 [ ] Define a Debian (amd64) system in linode in spread.yaml
 [ ] Add Debian-specific prepare code for packaging, etc
 [ ] Separate any Ubuntu specific code from the generic Debian code
 [ ] Blacklist tests that rely on confinement
 [ ] Blacklist tests that rely on Ubuntu-specific features (if any)

Ideally it would be good to add an Debian (unstable) image to https://github.com/zyga/spread-qemu-images so that development can be done against locally-running qemu target.