metrics: Scrape metrics from Kubernetes API server

Kubernetes exports a *lot* of metrics in Prometheus format.  I am not
sure what all is there, yet, but apparently several thousand time series
were added.

To allow anonymous access to the metrics, I added this RoleBinding:

```yaml
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRole
metadata:
  name: prometheus
rules:
- apiGroups:
  - ""
  resources:
  - nodes/metrics
  verbs:
  - get
- nonResourceURLs:
  - /metrics
  verbs:
  - get
```

roles/vmagent/files/ca-certs/kube-root-ca.crt

@@ -0,0 +1 @@
+../../../../kube-root-ca.crt

roles/vmagent/tasks/deploy.yml

@@ -46,6 +46,22 @@
   - config
   - scrape-config
 
+- name: ensure additional ca certificates are installed
+  copy:
+    src: '{{ item }}'
+    dest: /etc/victoria-metrics/{{ item | basename }}
+    mode: u=rw,go=r
+    owner: root
+    group: root
+  with_fileglob:
+  - ca-certs/{{ inventory_hostname }}/*.crt
+  - ca-certs/*.crt
+  notify:
+  - reload vmagent
+  tags:
+  - cert
+  - scrape-config
+
 - name: ensure vmagent starts at boot
   service:
     name: vmagent