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/tasks/install.yml

@@ -1,11 +1,4 @@
-- name: ensure vmagent systemd unit is installed
-  copy:
-    src: vmagent.service
-    dest: /etc/systemd/system/vmagent.service
-    mode: '0644'
-  notify:
-  - reload systemd
-  - restart vmagent
-  tags:
-  - service
-  - systemd
+- name: ensure podman is installed
+  package:
+    name: podman
+    state: present