Drop linux capabilities and rework users/groups

* Drop linux capabilities for unprivileged containerized
  worlkoads Kargo configures for deployments.
* Configure required securityContext/user/group/groups for kube
  components' static manifests, etcd, calico-rr and k8s apps,
  like dnsmasq daemonset.
* Rework cloud-init (etcd) users creation for CoreOS.
* Fix nologin paths, adjust defaults for addusers role and ensure
  supplementary groups membership added for users.
* Add netplug user for network plugins (yet unused by privileged
  networking containers though).
* Grant the kube and netplug users read access for etcd certs via
  the etcd certs group.
* Grant group read access to kube certs via the kube cert group.
* Remove priveleged mode for calico-rr and run it under its uid/gid
  and supplementary etcd_cert group.
* Adjust docs.
* Align cpu/memory limits and dropped caps with added rkt support
  for control plane.

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>

roles/etcd/templates/etcd-docker.service.j2

@@ -14,8 +14,12 @@ ExecStart={{ docker_bin_dir }}/docker run --restart=on-failure:5 \
 -v /etc/ssl/certs:/etc/ssl/certs:ro \
 -v {{ etcd_cert_dir }}:{{ etcd_cert_dir }}:ro \
 -v /var/lib/etcd:/var/lib/etcd:rw \
+{% for c in etcd_drop_cap %}
+--cap-drop={{ c }} \
+{% endfor %}
 --memory={{ etcd_memory_limit|regex_replace('Mi', 'M') }} --cpu-shares={{ etcd_cpu_limit|regex_replace('m', '') }} \
 --name={{ etcd_member_name | default("etcd") }} \
+-u {{ etcd_user_id }}:{{ etcd_group_id }} --group-add {{ etcd_cert_group_id }} \
 {{ etcd_image_repo }}:{{ etcd_image_tag }} \
 {% if etcd_after_v3 %}
 {{ etcd_container_bin_dir }}etcd

roles/etcd/templates/etcd-rkt.service.j2

@@ -8,6 +8,9 @@ Restart=on-failure
 RestartSec=10s
 TimeoutStartSec=0
 LimitNOFILE=40000
+User=root
+Group={{ etcd_group_id }}
+SupplementaryGroups={{ etcd_cert_group_id }}
 
 ExecStart=/usr/bin/rkt run \
 --uuid-file-save=/var/run/etcd.uuid \
@@ -20,6 +23,11 @@ ExecStart=/usr/bin/rkt run \
 --set-env-file=/etc/etcd.env \
 --stage1-from-dir=stage1-fly.aci \
 {{ etcd_image_repo }}:{{ etcd_image_tag }} \
+{% for c in etcd_drop_cap %}
+--caps-remove=CAP_{{ c.upper() }} \
+{% endfor %}
+--memory={{ etcd_memory_limit }} --cpu={{ etcd_cpu_limit }} \
+--user={{ etcd_user_id }} --group={{ etcd_group_id }} \
 --name={{ etcd_member_name | default("etcd") }}
 
 ExecStartPre=-/usr/bin/rkt rm --uuid-file=/var/run/etcd.uuid