When systemd-python is not installed, systemd notification falls back to
using the systemd-notify for service notification. This cannot be used
however unless the unit has NotifyAccess=all set.
The particular use case for this is when Salt is installed using pip. We
don't put systemd-python into the requirements.txt because we can't be
sure that the minion supports systemd, so pip installs won't necessarily
have systemd-python available.
