Rudder

This will take place along with the refactoring of system policies.
The specs are:

- store in settings policy_servers_allowed_networks,ou=Application Properties,cn=rudder-configuration a json alike:

{

"root":
[
{ "name": "the first allowed netwok", "address": "192.168.2.0/32" }, 
{ "name": "an other allowed netwok!", "address": "192.168.54.42" }, 
...
]
, 

"b1e0e408-083f-4061-974e-5a76677ad2b1":
[
{ "name": "allowed network for the relay", "address": "192.168.99.0/32" }, 

...
]

}

- create a system variable ALLOWED_NETWORKS that contains, for a given policy server, its allowed network in a JSON like:

[
{ "name": "allowed network #1 for root", "address": "192.168.2.0/32" }, 
{ "name": "allowed network #2 for root", "address": "192.168.54.42" }, 
...
]

- keep API as they are for now, we will be able to add name/description latter on.

Finally, we already had a system variable for allowed networks (`AUTHORIZED_NETWORKS`), we keep it for now.
We evolved the setting name to rudder_policy_server and store a JSON array of policy server (for now, only id and allowed networks)

After some discussion, we chose to rename AUTHORIZED_NETWORKS to ALLOWED_NETWORKS for consistency.