Launch the lxd images folowing the pattern ubuntu:${VERSION_ID}
The images installed like images:ubuntu/18.04 are presenting dependency
problemes. Use the same pattern for all the tests to define the image to
use ubuntu:${VERSION_ID} getting VERSION_ID from /etc/os-release
The introduction of sandbox/cgroup package has allowed some unification
and cleanup of the cgroup code. The remaining element was the freezer
operation used by snap-update-ns.
This patch moves the freezer code to the sandbox/cgroup package.
Existing freeze/thaw functions became public, as did the mocking helper
to control freezing. Minimal documentation for the affected functions
was added as well.
Signed-off-by: Zygmunt Krynicki <email address hidden>