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.

plugins/guests/freebsd/cap/configure_networks.rb

@@ -27,7 +27,7 @@ module VagrantPlugins
             end
 
             entry  = TemplateRenderer.render("guests/freebsd/network_#{network[:type]}",
-                                            options: network)
+                                            options: network, ifname: ifname)
 
             # Write the entry to a temporary location
             temp = Tempfile.new("vagrant")

templates/guests/freebsd/network_dhcp.erb

@@ -1,3 +1,3 @@
 #VAGRANT-BEGIN
-ifconfig_em<%= options[:interface] %>="DHCP"
+ifconfig_em<%= ifname %>="DHCP"
 #VAGRANT-END

templates/guests/freebsd/network_static.erb

@@ -1,3 +1,3 @@
 #VAGRANT-BEGIN
-ifconfig_em<%= options[:interface] %>="inet <%= options[:ip] %> netmask <%= options[:netmask] %>"
+ifconfig_<%= ifname %>="inet <%= options[:ip] %> netmask <%= options[:netmask] %>"
 #VAGRANT-END