[BUG] Cannot install only node_exporter

[n-rodriguez]

Your setup

Formula commit hash / release tag

Versions reports (master & minion)

Pillar / config used

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Bug details

Describe the bug

It fails with

       ----------
                 ID: prometheus-archive-install-prerequisites
           Function: file.directory
               Name: /var/lib/prometheus
             Result: False
            Comment: User prometheus is not available Group prometheus is not available
            Started: 19:23:19.133033
           Duration: 2.279 ms
            Changes:   
       ----------
                 ID: prometheus-archive-install-node_exporter
           Function: file.directory
               Name: /opt/prometheus/node_exporter-v0.18.1
             Result: False
            Comment: One or more requisite failed: prometheus.archive.install.prometheus-archive-install-prerequisites
            Started: 19:23:19.135989
           Duration: 0.006 ms
            Changes: 

because of https://github.com/saltstack-formulas/prometheus-formula/blob/master/prometheus/archive/install.sls#L16

Steps to reproduce the bug

Expected behaviour

Attempts to fix the bug

Additional context

[n-rodriguez]

@noelmcloughlin thank you!