Fix indentation of collector arguments in node_exporter systemd unit

[cod3mas0n]

Fixes #886

Changes

• Restructured the Jinja2 loop that emits collector flags so that every argument is emitted with a consistent 4-space indent.
• Simplified the if collector is mapping / else conditions and removed the problematic whitespace-control markers that caused the first line of a mapping collector to lose its indentation.

Before

ExecStart=/usr/local/bin/node_exporter \
    '--collector.systemd' \
'--collector.textfile' \
    '--collector.textfile.directory=/var/lib/node_exporter' \
'--collector.filesystem' \
    '--collector.filesystem.ignored-fs-types=^(sys|proc|auto)fs$' \
    '--collector.filesystem.ignored-mount-points=^/(sys|proc|dev)($|/)' \
    '--web.listen-address=0.0.0.0:9100' \
    '--web.telemetry-path=/metrics'

After

ExecStart=/usr/local/bin/node_exporter \
    '--collector.systemd' \
    '--collector.textfile' \
    '--collector.textfile.directory=/var/lib/node_exporter' \
    '--collector.filesystem' \
    '--collector.filesystem.ignored-fs-types=^(sys|proc|auto)fs$' \
    '--collector.filesystem.ignored-mount-points=^/(sys|proc|dev)($|/)' \
    '--web.listen-address=0.0.0.0:9100' \
    '--web.telemetry-path=/metrics'