Remove support for hostmanager and hostsupdater Vagrant plugins

[aldavigdis]

As hostsupdater is no longer maintained and hostsmanager has dependency issues, removing them as optional alternatives to goodhosts would streamline a lot of things.

Plugin checks in the Vagrantfile are non-functioning as Vagrant.has_plugin? will always return a falsey value as it is currently being used, so that Vagrant will default on using and installing goodhosts no matter if the other plugins as installed.

This also provides opportunities to revamp the Vagrantfile in a comprehensive way. (Subject to separate issue ticket.)

Describe alternatives you've considered

Additional context

It would be useful to have some info on how many users are using the hostsupdater and hostsmanager plugins and if this would potentially cause issues for them and how severe they are.

This was discussed in #2638.

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[Mte90]

Right now we don't know how many people uses them but honestly I think that if have the alert to install the plugin as mandatory it will simple for them to switch to that and avoid so many troubles.