Add keepalived element

This element will be needed for HA deployments

Change-Id: Ice66cc490abaf54109c0678296055d111ef69f4c

elements/keepalived/README.md

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+Element to install keepalived
+
+Configuration
+-------------
+
+keepalived:
+  keepalive_interface: eth0
+    - interface used for VRRP traffic
+virtual_ips:
+  - you can define one or more virtual IPs including IPv6:
+- ip: 192.0.2.254/24
+  interface: eth0
+- ip: fe80::5cc1:afff:fe58:143b/64
+  interface: br-ctlplane

elements/keepalived/install.d/76-keepalived

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+#!/bin/bash
+
+set -eux
+
+install-packages keepalived
+
+if [ "$DIB_INIT_SYSTEM" == "systemd" ] ; then
+  systemctl enable keepalived.service
+fi

elements/keepalived/os-config-applier/etc/keepalived/keepalived.conf

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+vrrp_script chk_haproxy {
+      script "killall -0 haproxy"
+      interval 2
+      weight 2
+}
+
+vrrp_instance VI_1 {
+      interface {{keepalived.keepalive_interface}}
+      state MASTER
+      virtual_router_id 51
+      priority {{keepalived.priority}} # 101 on master, 100 on backup
+      virtual_ipaddress {
+           {{#virtual_ips}}
+           {{ip}} dev {{interface}}
+           {{/virtual_ips}}
+      }
+      track_script {
+           chk_haproxy
+      }
+}

elements/keepalived/os-refresh-config/post-configure.d/15-keepalived

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+#!/bin/bash
+set -eux
+
+service keepalived restart