Cleanup .vagrant directory when 'netlab down --cleanup' is used

[jbemmel]

When using Vagrant boxes, a .vagrant directory is created. Disk images for the vms get removed, but the directory itself remains

If there are concerns around deleting non-Netlab related files, we could consider renaming .vagrant to .netlab.vagrant

[ipspace]

Nobody complained in the four years we're using Vagrant, so I guess this is not a high priority (as opposed to all the visible stuff Ansible and containerlab leave behind)