Update driver.rb

Fixing https://github.com/fgrehm/vagrant-lxc/issues/353 issue

lib/vagrant-lxc/driver.rb

@@ -172,6 +172,11 @@ module Vagrant
       end
 
       def remove_bridge(bridge_name)
+        if bridge_name == "lxcbr0"
+           @logger.info "Skipping removal system bridge #{bridge_name}"
+           return
+        end
+
         @logger.info "Checking whether bridge #{bridge_name} exists"
         brctl_output = `ifconfig -a | grep -q #{bridge_name}`
         return if $?.to_i != 0