cg: add --cgroup-root option

The motivation for this is to be able to restore containers into cgroups other
than what they were dumped in (if, e.g. they might conflict with an existing
container). Suppose you have a container in:

memory:/mycontainer
cpuacct,cpu:/mycontainer
blkio:/mycontainer
name=systemd:/mycontainer

You could then restore them to /mycontainer2 via --cgroup-root /mycontainer2.
If you want to restore different controllers to different paths, you can
provide multiple arguments, for example, passing:

--cgroup-root /mycontainer2 --cgroup-root cpuacct,cpu:/specialcpu \
--cgroup-root name=systemd:/specialsystemd

Would result in things being restored to:

memory:/mycontainer2
cpuacct,cpu:/specialcpu
blkio:/mycontainer2
name=systemd:/specialsystemd

i.e. a --cgroup-root without a controller prefix specifies the new default root
for all cgroups.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
tests: add a test for --cgroup-root

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>