zsh-completion: systemctl: list template units only as needed

zsh-completion: systemctl: list template units only as needed

Currently the completion adds template units for commands such as
is-active, is-failed, is-enabled, status, show and others.

At the same time systemctl barfs at us, since an instanced template unit
is needed. Follow the example list from bash-completion as to which
commands should not list template units.

Note: The above is observed regardless of DefaultInstance.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>