fstrim: run service and timer only if /etc/fstab is present

fstrim: run service and timer only if /etc/fstab is present

The timer and service units run `fstrim --fstab`, which strictly
depends on `/etc/fstab` being present in the OS. This adds relevant
condition statements to the units, in order to skip them and avoid
runtime failures.

Ref: https://github.com/karelzak/util-linux/issues/673#issuecomment-409246816
Ref: https://github.com/coreos/bugs/issues/2591
Ref: https://github.com/coreos/fedora-coreos-tracker/issues/468

Signed-off-by: Luca BRUNO <luca.bruno@coreos.com>