Add user check when assigning a permission

1. create the following chain:

   - Context.FetchDocument
   - Document.AddPermission:
       permission: Read
       username: kvjeniozernofiuzehrt
       acl: local
       blockInheritance: "false"
       notify: "false"
   

2. create a button in User actions to trigger the chain on a current document
3. synchronize the project with Nuxeo server
4. check there is no user named kvjeniozernofiuzehrt
5. create a Document
6. trigger the chain
7. check the permission tab, a Read permission has been give to user kvjeniozernofiuzehrt

This is actually done here:
https://github.com/nuxeo/nuxeo/blob/master/nuxeo-features/nuxeo-automation/nuxeo-automation-core/src/main/java/org/nuxeo/ecm/automation/core/operations/document/AddPermission.java

The code does not check that the username corresponds to an entry in the user directory. Same problem for group directory as well.