Allow boostrap with Buster

- add script and Vagrantfile code to preseed salt minion keys
- add needed bootstrap_options for Python 3 support
- add grains files and populate those of the ldap and web node
- add script to fix systemd-networkd setup with Debian libvirt Vagrant
  images
- switch from VirtualBox to libvirt

salt/grains/ldap

@@ -0,0 +1,4 @@
+roles:
+  - vagrant
+  - ldapserver
+  - gnuviechadmin.gvaldap

salt/grains/web

@@ -0,0 +1,7 @@
+roles:
+  - vagrant
+  - ldapclient
+  - webserver
+  - gnuviechadmin.gvaweb
+nginx:
+  default_servername: web.local