fstrim shouldn't run inside a container

fstrim shouldn't run inside a container

Container type implies the following products:
openvz	OpenVZ/Virtuozzo
lxc	Linux container implementation by LXC
lxc-libvirt	Linux container implementation by libvirt
systemd-nspawn	systemd's minimal container implementation, see systemd-nspawn(1)
docker	Docker container manager
podman	Podman container manager
rkt	rkt app container runtime
wsl	Windows Subsystem for Linux

References:
https://www.freedesktop.org/software/systemd/man/systemd.unit.html
https://www.freedesktop.org/software/systemd/man/systemd-detect-virt.html#

Fix: #840

Signed-off-by: Eric Desrochers <eric.desrochers@canonical.com>