Set the mysql version for Ubuntu 18.04

The current mysql cookbook doesn't know the version it ships with

site-cookbooks/backup/recipes/default.rb

@@ -64,6 +64,7 @@ include_recipe 'logrotate'
 # Install MySQL client (includes mysqldump)
 mysql_client 'default' do
   action :create
+  version '5.7' if node[:platform_version].to_f == 18.04
   not_if { node["backup"]["mysql"]["databases"].empty? }
 end