Removed the vagrant-hostmanger plugin

This is not needed for the virtualbox environment since we hard code the IP address in the Vagrantfile for each VM.

The hostmanger plugin is required when using digital ocean droplets instead of virtualbox.

Digital ocean droplets are used in our snap-ci environment and can be useful for developers using Qubes or other hosts that do not support virtualbox.

When using digital ocean droplets the IP address of the VPS can change when you destroy and rebuild the vps. The hostmanger plugin was used with the ansible skip-tag `static-hosts` to configure the `/etc/hosts` entries.
@dolanjs Can we remove these plugins too? Seems like this section doesn't need to be here anymore.