Fix how Vagrant assigns cli arguments for the create command

This commit inlines the flag assignments so that arguments are properly
assigned to flags rather than arguments to the subcommand.

plugins/providers/docker/action/network.rb

@@ -22,7 +22,7 @@ module VagrantPlugins
               # `docker network create` doesn't care about these options
               next
             else
-              cli_opts.concat(["--#{opt}", value])
+              cli_opts.concat(["--#{opt}=#{value.to_s}"])
             end
           end