Add multi user assignement on one machine for ldap, and a limit of users per machines. (#497)

* First part of integrating join table for machines and users

* Second part of adding manyusers to one machine

* Add ldap username to users models

* Refactor test to correspond with the new use/machine system

* Add test for ldap multi user
Add front-end part
Add refactor of fake plaza

* Last part of assigning multiple users to a machine with ldap.
Start application with the administrator login, because plaza don't
support multi users.

* Rebase on master

* Force plaza to fork itself with Administrator user.
Rewording

* Add users on machine details

* Solve test problems