(maint) permit PlatformTagContainer class

This commit adds the PlatformTagContainer class to the
HostsFileParser module so that tests can be re-run. This
class was added to the 5.x series in a previous commit [1]
but was not added to the 4.x branch. For context see issue
1753 [2].

[1]: 45f0bf9
[2]: #1753

lib/beaker/options/hosts_file_parser.rb

@@ -4,6 +4,7 @@ module Options
     module HostsFileParser
       PERMITTED_YAML_CLASSES = [
         'Beaker',
+        'Beaker::DSL::TestTagging::PlatformTagConfiner',
         'Beaker::Logger',
         'Beaker::Options::OptionsHash',
         'Beaker::Platform',