Feature/sdn-notification

This patch introduces async notifications to the SDN application once a flow is created, deleted, updated or a creation just failed.

Each NCL Provisioner user would have its own client, managed by the ClientManager class. Therefore, it contains a map storing the association between the username and its instantiated client. If the user does not have any instantiated client, the ClientManager calls the ClientFactory to create one.

The NotificationClient needs an url, This url is contained in the user registry. Each user field contains a username, password and url.

The most important modification regarding security stuff is the introduction of a ThreadLocal in the spring security filter. It needs to store the username, so the NCL Provisioner is able to access it.