Fix for lxc on Ubuntu 16.10
Fix for lxc on Ubuntu 16.10

It looks that on Ubuntu 16.10 lxc containers cgroups may contain
systemd slices:

11:devices:/lxc/x1/system.slice/snapd.service
10:memory:/lxc/x1/system.slice/snapd.service
9:hugetlb:/lxc/x1
8:perf_event:/lxc/x1
7:cpuset:/lxc/x1
6:pids:/lxc/x1/system.slice/snapd.service
5:freezer:/lxc/x1
4:cpu,cpuacct:/lxc/x1/system.slice/snapd.service
3:net_cls,net_prio:/lxc/x1
2:blkio:/lxc/x1/system.slice/snapd.service
1:name=systemd:/lxc/x1/system.slice/snapd.service

this fix uses as container id only the first path after /lxc/ instead of
everything. It looks safe since `/` is forbidden as containerid

angeloc/sysdig,angeloc/sysdig,angeloc/sysdig,angeloc/sysdig,angeloc/sysdig