[master] use systemctl is-active to check for containerd

[andrewhsu]

Without this PR, unable to detect an active containerd.io package:

$ systemctl status containerd | head -3
● containerd.service - containerd container runtime
   Loaded: loaded (/lib/systemd/system/containerd.service; enabled; vendor preset: enabled)
   Active: active (running) since Wed 2018-08-22 23:44:45 UTC; 6h ago
$ make -pn -f containerd.mk | grep -e ^CONTAINERD_SOCK
make: *** No targets.  Stop.
CONTAINERD_SOCK := /var/run/docker/containerd/docker-containerd.sock

With this PR, allows proper detection:

$ make -pn -f containerd.mk | grep -e ^CONTAINERD_SOCK
make: *** No targets.  Stop.
CONTAINERD_SOCK := /var/run/containerd/containerd.sock

[seemethere]

LGTM

[seemethere]

This will also need to be made to the 18.09 branch

[andrewhsu]

@seemethere ok, i'll take care of it