(maint) Refer to new Beaker::Host::Generator::Data constant
(maint) Refer to new Beaker::Host::Generator::Data constant

Previously, trying to call `beaker-hostgenerator --list` would result
in:

    uninitialized constant Beaker::Host::Generator::Utils::GenConfig (NameError)

This commit updates the code to use the new constant name.

puppetlabs/beaker-hostgenerator