Pin Vagrant box version

The existing night-rally fixtures rely on a Ansible versions <2.8.
Pin box version to ensure it uses Vagrant images that come with
Ansible <2.8.

Relates elastic#116

Author: dliappis

night_rally/fixtures/ansible/Vagrantfile

@@ -11,6 +11,8 @@ end
 
 base_ip = '192.168.180.10'
 boxname = "elastic/ubuntu-16.04-x86_64"
+# Pin an older image to ensure Ansible <2.8
+box_version = "20190623.0.0"
 
 default_cpus = 2
 default_target_node_count = 3
@@ -22,11 +24,11 @@ default_target_memory = ENV.fetch("VAGRANT_TARGET_MEMORY", "5120")
 install_build_requirements = ENV["VAGRANT_ENABLE_BUILD"].to_s.downcase=="true"
 domain_name = "benchmarks.ci.vagrant"
 
-nodes = [{ hostname: "coordinator", domain: domain_name, autostart: true, box: boxname, memory: default_load_driver_memory }]
+nodes = [{ hostname: "coordinator", domain: domain_name, autostart: true, box: boxname, box_version: box_version, memory: default_load_driver_memory }]
 
 # target-nodes
 for i in 1..default_target_node_count
-  nodes += [{ hostname: "target%02d" % [i], domain: domain_name, autostart: true, box: boxname, memory: default_target_memory.to_i }]
+  nodes += [{ hostname: "target%02d" % [i], domain: domain_name, autostart: true, box: boxname, box_version: box_version, memory: default_target_memory.to_i }]
 end
 
 rally_repo = ENV.fetch("RALLY_REPO", "https://github.com/elastic/rally.git")
@@ -194,6 +196,7 @@ Vagrant.configure(2) do |config|
 
     config.vm.define node[:hostname], autostart: (node[:autostart] || false) do |node_config|
       node_config.vm.box = node[:box] || boxname
+      node_config.vm.box_version = box_version
       node_config.vm.hostname = fqdn
       node_config.vm.network :private_network, ip: ip_address.to_s