r/v-m: Add role for Victoria Metrics

The *victoria-metrics* role deploys a single-server instance of the
Victoria Metrics time series database server.  It installs the selected
version by downloading the binary release from Github and copying it to
`/usr/local/sbin` on the managed node.  Scrape configuration is optional
and can be specified with the `scrape_configs` variable.

roles/victoria-metrics/files/victoria-metrics.service

@@ -0,0 +1,15 @@
+[Unit]
+Description=VictoriaMetrics
+
+[Service]
+Type=exec
+Environment=httpListenAddr=[::1]:8428
+Environment=storageDataPath=/var/lib/victoria-metrics/data
+Environment=loggerDisableTimestamps=true
+EnvironmentFile=-/etc/sysconfig/victoria-metrics
+ExecStart=/usr/local/sbin/victoria-metrics -enableTCP6 -envflag.enable
+User=victoriametrics
+LimitNOFILE=1048576
+
+[Install]
+WantedBy=multi-user.target