Update nr_open limit value in the amphora

Using haproxy 2.x, the ideal rlimit value for nr_open is close to
connection_limit * 2.5 (see compute_ideal_* in src/haproxy.c).
Set this limit to 2,600,000 in the amphora to support a loadbalancer
with maxconn 1M.

This prevents the following warning messages when launching/reloading
haproxy:

* "Cannot raise FD limit to 2375058, limit is 2097152."
* "FD limit (2097152) too low for maxconn=950000/maxsock=2375058. Please
  raise 'ulimit-n' to 2375058 or more to avoid any trouble."

Change-Id: I6251cd17bd6fa9faf5109e50c2190dda3614908d
(cherry picked from commit 4174f4a5a4)
(cherry picked from commit f55376bf5d)
(cherry picked from commit c177987f81)
(cherry picked from commit e4300558e1)

elements/haproxy-octavia/post-install.d/20-haproxy-tune-kernel

@@ -13,8 +13,8 @@ sysctl-write-value net.core.somaxconn 65534
 sysctl-write-value net.ipv4.tcp_synack_retries 3
 sysctl-write-value net.core.netdev_max_backlog 100000
 # This should allow HAProxy maxconn to be 1,000,000
-sysctl-write-value fs.file-max 2097152
-sysctl-write-value fs.nr_open 2097152
+sysctl-write-value fs.file-max 2600000
+sysctl-write-value fs.nr_open 2600000
 
 # It's ok for these to fail if conntrack module isn't loaded
 sysctl-write-value net.ipv4.netfilter.ip_conntrack_tcp_timeout_time_wait 5 || true

octavia/amphorae/backends/agent/api_server/templates/systemd.conf.j2

@@ -30,7 +30,7 @@ Type=notify
 
 KillMode=mixed
 Restart=always
-LimitNOFILE=2097152
+LimitNOFILE=2600000
 
 [Install]
 WantedBy=multi-user.target

releasenotes/notes/fix-nr_open-limit-value-7f475c3e301a608d.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Increase the limit value for nr_open and file-max in the amphora, the new
+    value is based on what HAProxy 2.x is expecting from the system with the
+    greatest maxconn value that Octavia can set.