Fix NoMethodError on vagrant up w/o floating_ip

Hi.
In my enviroment, without floating_ip settings in Vagrantfile, 'vagrant up'  hits an error and fails.

create_server.rb code looks requires floating_ip but that should be optional.
Could you please review and merge this patch?

error:
```
/Users/xxx/.vagrant.d/gems/gems/vagrant-openstack-plugin-0.6.0/lib/vagrant-openstack-plugin/action/create_server.rb:100:in `block in call': undefined method `to_sym' for nil:NilClass (NoMethodError)
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/util/retryable.rb:17:in `retryable'
        from /Users/xxx/.vagrant.d/gems/gems/vagrant-openstack-plugin-0.6.0/lib/vagrant-openstack-plugin/action/create_server.rb:86:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/warden.rb:34:in `call'
        from /Users/xxx/.vagrant.d/gems/gems/vagrant-openstack-plugin-0.6.0/lib/vagrant-openstack-plugin/action/warn_networks.rb:14:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/builtin/set_hostname.rb:16:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/warden.rb:34:in `call'
        from /Users/xxx/.vagrant.d/gems/gems/vagrant-openstack-plugin-0.6.0/lib/vagrant-openstack-plugin/action/sync_folders.rb:17:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/builtin/provision.rb:76:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/warden.rb:34:in `call'
        from /Users/xxx/.vagrant.d/gems/gems/vagrant-openstack-plugin-0.6.0/lib/vagrant-openstack-plugin/action/connect_openstack.rb:56:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/warden.rb:95:in `block in finalize_action'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/builder.rb:116:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/runner.rb:69:in `block in run'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/util/busy.rb:19:in `busy'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/runner.rb:69:in `run'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/builtin/call.rb:51:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/builtin/config_validate.rb:25:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/builtin/handle_box.rb:56:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/builtin/handle_box_url.rb:9:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/warden.rb:34:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/builder.rb:116:in `call'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/runner.rb:69:in `block in run'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/util/busy.rb:19:in `busy'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/action/runner.rb:69:in `run'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/machine.rb:157:in `action'
        from /Applications/Vagrant/embedded/gems/gems/vagrant-1.5.2/lib/vagrant/batch_action.rb:72:in `block (2 levels) in run'
```