Respect spaces in ARGS passed to systemd unit

Currently with a drop file setting of say

```
Environment=ARGS="--add *"
```

this calls `lpadmincern '--add *'`

Previously with the perl version this was accepted but in fact
lpadmincern did nothing.

With the python version this quite correctly gives a parameter
validation error.

From `man systemd.service`

> Basic environment variable substitution is supported. Use "${FOO}" as
part of a word, or as a word of its own, on the command line, in which
case it will be replaced by the value of the environment
variable including all whitespace it contains, resulting in a single
argument. Use "$FOO" as a separate word on the command line, in which
case it will be replaced by the value of the environment variable
split at whitespace, resulting in zero or more arguments. For this type
of expansion, quotes are respected when splitting into words, and
afterwards removed.

It was worth coming to work today.
13 jobs for envs in 2 minutes and 31 seconds (queued for 28 seconds)