B #2824 fix LXD container monitoring (#2840)
B #2824 fix LXD container monitoring (#2840)

* handle exceptions in get_memory()
* add option to set cgroup prefix to the container path via the environment variable `LXC_CGROUP_PREFIX`

In case of a more complex cgroup configuration the containers cgroup could be placed in separate slice instead of default root cgroup. In this case the defining LXC_CGROUP_PREFIX="system.slice/" (note the trailing slash) in the environment will help vmm/lxd/poll to construct correct path the needed *memory* and *cpu,cpuacct* cgroups

```bash
su - oneadmin -c 'echo "export LXC_CGROUP_PREFIX=system.slice" >>~/.bashrc'
```

baby-gnu/one,baby-gnu/one,OpenNebula/one,atodorov-storpool/one,OpenNebula/one,atodorov-storpool/one,atodorov-storpool/one,baby-gnu/one,baby-gnu/one,atodorov-storpool/one,OpenNebula/one,OpenNebula/one,baby-gnu/one,OpenNebula/one,atodorov-storpool/one,OpenNebula/one,baby-gnu/one,OpenNebula/one,baby-gnu/one,atodorov-storpool/one,baby-gnu/one,atodorov-storpool/one,OpenNebula/one,atodorov-storpool/one,OpenNebula/one,atodorov-storpool/one