driver: Do not show a stacktrace to the user if no network.hwaddr is present on container configs

This is enough to close GH-266 since Vagrant will take care of showing
an error informing the user that it was unable to connect to the
machine.

lib/vagrant-lxc/driver.rb

@@ -44,7 +44,11 @@ module Vagrant
       end
 
       def mac_address
-        @mac_address ||= config_string.match(/^lxc\.network\.hwaddr\s*+=\s*+(.+)$/)[1]
+        return @mac_address if @mac_address
+
+        if config_string =~ /^lxc\.network\.hwaddr\s*+=\s*+(.+)$/
+          @mac_address = $1
+        end
       end
 
       def config_string