Add restriction on SystemLogin usage

Nuxeo Runtime provides a way to do a SystemLogin via Framework.login().
This is used to be able to login as a system account without having to provide a login/password.
This is used in Nuxeo :

• when current user needs to gain "root privileges" : typically usage of UnrestrictedSessionRunner
• when an unauthenticated thread needs to access the repository : typically a async listener

This system login can also be used for RMI access : this is the case for Nuxeo Shell that connect to a remote Nuxeo instance as system.

In order to better handle restrictions on this SystemLogin we introduce :

• a identifier for Nuxeo Runtime instances :

This identifier will be by default a automatically generated VMID, but can be set explicitly via nuxeo.properties (org.nuxeo.runtime.instance.id)

• properties to configure restrictions for SystemLogin

• org.nuxeo.systemlogin.restrict : true/false (default true) ; turns on/off restrictions

• org.nuxeo.systemlogin.trusted.instances : comma separated list of trusted runtime instances (default : empty)

When restrictions are on, SystemLogin calls will be granted only :

• when it comes from the same JVM (necessary for have UnrestrictedSessionRunner working)

• when it comes from a trusted host

Turning off restrictions will result in a fallback to the old behavior : always grant SystemLogin

NB : The trusted hosts are identfiied by a simple id, but technically this is a shared secret between the clients and the server, so this should be enough for most cases.

Direct impacts on Nuxeo are :

• Nuxeo Shell won't be able to connect as system anymore with the default Nuxeo configuration
  => need to update the NXShell to force login

• Stateless/Statefull package needs to be updated