Add support for ed25519 ssh keys for docker

beaker-docker uses net-ssh for interaction with containers. Ubuntu 18.04
defaults to ed25519 keys. We need additional gems for this keytype. More
information is present at
voxpupuli/modulesync_config#478. This should be
added to the modulesync_config repo / pdk config.

Gemfile

@@ -43,6 +43,9 @@ group :system_tests do
   gem "beaker-pe",                                                               require: false
   gem "beaker-hostgenerator"
   gem "beaker-rspec"
+  gem 'rbnacl', '~> 4',                     :require => false if RUBY_VERSION >= '2.2.6'
+  gem 'rbnacl-libsodium',                   :require => false if RUBY_VERSION >= '2.2.6'
+  gem 'bcrypt_pbkdf',                       :require => false
 end
 
 puppet_version = ENV['PUPPET_GEM_VERSION']