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/templates/victoria-metrics.sysconfig.j2

@@ -0,0 +1,7 @@
+retentionPeriod={{ vm_retention_period }}
+{% if vm_self_scrape|bool %}
+selfScrapeInterval={{ vm_self_scrape_interval }}
+{% endif %}
+{% if vm_config is not none %}
+promscrape_config=/etc/victoria-metrics/scrape.yml
+{% endif %}