r/vmagent: Rework as container deployment

Like the _blackbox-exporter_ role, the _vmagent_ role now deploys
`vmagent` as a container.  This simplifies the process considerably,
eliminating the download/transfer step.

While refactoring this role, I also changed how the trusted CA
certificates are handled.  Rather than copy files, the role now expects
a `vmagent_ca_certs` variable.  This variable is a mapping of
certificate name (file name without extension) to PEM contents.  This
allows certificates to be defined using normal host/group variables.

roles/vmagent/templates/vmagent.container.j2

@@ -0,0 +1,26 @@
+[Unit]
+Description=Victoria Metrics vmagent
+After=network.target
+Wants=network.target
+
+[Container]
+Image={{ vmagent_container_image_name }}:{{ vmagent_container_image_tag }}
+Pull=never
+Exec=-enableTCP6 -envflag.enable
+Environment=loggerDisableTimestamps=true
+EnvironmentFile=/etc/sysconfig/vmagent
+Mount=type=bind,source=/etc/victoria-metrics,target=/etc/victoria-metrics,readonly=true
+Mount=type=bind,source=/var/lib/victoria-metrics,target=/var/lib/victoria-metrics,chown=true,relabel=shared
+WorkingDir=/var/lib/victoria-metrics/vmagent
+ReadOnly=yes
+ReadOnlyTmpfs=yes
+NoNewPrivileges=yes
+User=212
+Group=212
+Network=host
+
+[Service]
+Restart=always
+RestartSec=1s
+ExecReload=/usr/bin/podman kill --cidfile=%t/%N.cid -s HUP
+StateDirectory=victoria-metrics victoria-metrics/vmagent