Use a simple systemd timer for node-exporter-scripts

It has better debuggability than the flurry of little wrapped cron runners, and the scripts are more deploy-friendly.

roles/float-base/files/run-node-exporter-script.sh

-#!/bin/sh
-#
-# Execute a metrics-generating script and safely write its
-# output to the /var/lib/prometheus/node-exporter directory.
-#
-# Uses 'runcron' for locking, so don't wrap it in another
-# 'runcron' invocation!
-#
-
-if [ $# -ne 2 ]; then
-    echo "Usage: $0 <snippet-name> <script-path>"
-    exit 2
-fi
-
-script_name="$1"
-script_path="$2"
-output_dir="/var/lib/prometheus/node-exporter"
-
-test -d $output_dir || exit 0
-
-umask 022
-
-output_file="${output_dir}/${script_name}.prom"
-tmp_file="${output_file}.$$"
-trap "rm -f $tmp_file 2>/dev/null" EXIT INT TERM
-
-runcron --no-syslog --no-metrics --splay 60 --name "node-exporter-$script_name" -- \
-        "$script_path" > "$tmp_file"
-if [ $? -gt 0 ]; then
-    rm -f "$tmp_file" 2>/dev/null
-    exit 1
-else
-    mv -f "$tmp_file" "$output_file"
-fi
-
-exit $?