Connect to openstack network resource when needed

I was getting the following error when using `config.network` (as opposed to `config.networks`).

```
/Users/hnewton/.vagrant.d/gems/gems/vagrant-openstack-plugin-0.8.0/lib/vagrant-openstack-plugin/action/create_server.rb:63:in `block in call': undefined method `networks' for nil:NilClass (NoMethodError)
        from /Users/hnewton/.vagrant.d/gems/gems/vagrant-openstack-plugin-0.8.0/lib/vagrant-openstack-plugin/action/create_server.rb:61:in `each'
        from /Users/hnewton/.vagrant.d/gems/gems/vagrant-openstack-plugin-0.8.0/lib/vagrant-openstack-plugin/action/create_server.rb:61:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:34:in `call'
        from /Users/hnewton/.vagrant.d/gems/gems/vagrant-openstack-plugin-0.8.0/lib/vagrant-openstack-plugin/action/create_orchestration_stack.rb:21:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:34:in `call'
        from /Users/hnewton/.vagrant.d/gems/gems/vagrant-openstack-plugin-0.8.0/lib/vagrant-openstack-plugin/action/connect_openstack.rb:61:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/builtin/set_hostname.rb:16:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:34:in `call'
        from /Users/hnewton/.vagrant.d/gems/gems/vagrant-openstack-plugin-0.8.0/lib/vagrant-openstack-plugin/action/warn_networks.rb:14:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:34:in `call'
        from /Users/hnewton/.vagrant.d/gems/gems/vagrant-openstack-plugin-0.8.0/lib/vagrant-openstack-plugin/action/sync_folders.rb:17:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:34:in `call'
        from /Users/hnewton/.vagrant.d/gems/gems/vagrant-cachier-0.9.0/lib/vagrant-cachier/action/install_buckets.rb:14:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/builtin/provision.rb:80:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:34:in `call'
        from /Users/hnewton/.vagrant.d/gems/gems/vagrant-cachier-0.9.0/lib/vagrant-cachier/action/configure_bucket_root.rb:20:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:95:in `block in finalize_action'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/builder.rb:116:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/runner.rb:66:in `block in run'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/util/busy.rb:19:in `busy'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/runner.rb:66:in `run'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/builtin/call.rb:53:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/builtin/config_validate.rb:25:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/builtin/handle_box.rb:56:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/builder.rb:116:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/runner.rb:66:in `block in run'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/util/busy.rb:19:in `busy'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/action/runner.rb:66:in `run'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/machine.rb:196:in `action_raw'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/machine.rb:173:in `block in action'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/environment.rb:434:in `lock'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/machine.rb:161:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/machine.rb:161:in `action'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.6.3/lib/vagrant/batch_action.rb:82:in `block (2 levels) in run'
```