tree-wide: use container_uses_namespace() helper

tree-wide: use container_uses_namespace() helper

No functional changes.

Will be useful in future support for an isolated
user namespaces [1]. I have already played with
that locally and found that in the LXC codebase
we have a bunch of different ways to ensure if
a container uses user namespaces or not.

This commit contains a trivial conversion from
an open-coded version of the container_uses_namespace()
helper to an actual use of the helper.

[1] https://lpc.events/event/17/contributions/1569/

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>