Removes vagrant-hostmanager plugin from Vagrantfile (#1311)
Removes vagrant-hostmanager plugin from Vagrantfile (#1311)

The plugin is not used in the config anywhere, nor is the requirement
documented in our developer docs. Therefore let's remove it, because
first-time setup for SecureDrop VMs is a frustrating experience when
it's not nearly as simple as `vagrant up`.
heartsucker/securedrop,ageis/securedrop,micahflee/securedrop,micahflee/securedrop,conorsch/securedrop,micahflee/securedrop,conorsch/securedrop,ehartsuyker/securedrop,ageis/securedrop,garrettr/securedrop,micahflee/securedrop,garrettr/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,garrettr/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,garrettr/securedrop,conorsch/securedrop,ageis/securedrop,ageis/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop