Do not return an error if ifdown fails

Ubuntu versions prior to 16.04 always returned a successful exit status,
even if one tried to down an interface that does not exist. This
behavior changed in Ubuntu 16.04 to return an error. This commit
preserves the old behavior.

Fixes GH-7155

Author: sethvargo

plugins/guests/debian/cap/configure_networks.rb

@@ -42,7 +42,10 @@ def self.configure_networks(machine, networks)
             # each specifically, we avoid reconfiguring eth0 (the NAT interface) so
             # SSH never dies.
             interfaces.each do |interface|
-              comm.sudo("/sbin/ifdown eth#{interface} 2> /dev/null")
+              # Ubuntu 16.04+ returns an error when downing an interface that
+              # does not exist. The `|| true` preserves the behavior that older
+              # Ubuntu versions exhibit and Vagrant expects (GH-7155)
+              comm.sudo("/sbin/ifdown eth#{interface} 2> /dev/null || true")
               comm.sudo("/sbin/ip addr flush dev eth#{interface} 2> /dev/null")
             end