conf: introduce lxc.rootfs.managed

conf: introduce lxc.rootfs.managed

This introduces a new config key lxc.rootfs.managed which can be used to
indicate whether this LXC instance is managing the container storage. If LXC is
not managing the storage then LXC will not modify the container storage.
For example, an API call to c->destroy(c) will then run any destroy hooks but
will not destroy the actual rootfs (Unless, of course, the hook does so behind
LXC's back.).

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
CC: Wolfgang Bumiller <w.bumiller@proxmox.com>
CC: Stéphane Graber <stgraber@ubuntu.com>
CC: Serge Hallyn <serge@hallyn.com>
CC: 2xsec <dh48.jeong@samsung.com>