Fix Issue 4307: FreeBSD fix interface device names in /etc/rc.conf

https://github.com/mitchellh/vagrant/issues/4307

Fix details:
Pass network interface name to template renderer and use it
in static and dhcp templates to render /etc/rc.conf entries.

  modified:   plugins/guests/freebsd/cap/configure_networks.rb
  modified:   templates/guests/freebsd/network_dhcp.erb
  modified:   templates/guests/freebsd/network_static.erb

Verification scenario:
Check if /etc/rc.conf contains vtnet entries when paravirtualized
NIC devices are used in FreeBSD guest.
Fix network_dhcp template.
Merge pull request #4308 from Voxer/fix/master/issue4307

guest/freebsd: fix interface device names in /etc/rc.conf