On Debian and Ubuntu guests, when the new hostname of the guest box is set, the DHCP client (isc-dhcp-client) won't renew its DHCP lease with the new hostname. Executing "ifdown -a; ifup -a" to reinitialize all auto interfaces fixes this issue. Furthermore, as vagrant will now actually wait until a DHCP lease is acquired, it is guaranteed that the the correct domainname is set from DHCP before calling other provisioners like puppet, that rely on it. This fixes puppet/facter sometimes failing to find the fqdn fact on an LXC guest when the DHCP server is responding too slow.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.