Check global.enabled instead of machine.enabled

update_all.rb checks if @machine.config.hostmanager.enabled is set to true.
The default value of @enabled is set to false by config.rb.
If the Vagrantfile has a global value of true for enabled but it sets vm
specific hostmanager options, such as when specifying hostmanager.aliases,
then the @machine.config.hostmanager.enabled will default to false and
hostmanager will not think it is enabled.

Check @global_env.config_global.hostmanager.enabled instead.