Description of problem:
acl definitions are not enforced and could be bypassed by a user without write access to the cib

Version-Release number of selected component (if applicable):
RedHat Enterprise Linux 6.6
pcs --version = 0.9.123
pacemakerd --version = Pacemaker 1.1.11

How reproducible:
a user with a read-only role can assign any other existing roles to himself and then gain any kind of access from any role (rw access to the cib if this kind of role exist).

Steps to Reproduce:
1. create a role read-only
   pcs acl role create read-only description="Read only access" read xpath /cib
2. create a role admin
   pcs acl role create admin description="Admin access" write xpath /cib
3. create an account (local + pcs)
4. open a session with this roaccount account
5. add admin role to your account
   pcs acl role assign admin to rocluster
6. check new acl pushed as a read-only user
   pcs acl
  User: rocluster
    Roles: read-only admin
  Role: read-only
    Description: Read only access
    Permission: read xpath /cib (read-only-read)
  Role: admin
    Description: Admin access
    Permission: write xpath /cib (admin-write)
7. add/delete/modify anything

Actual results:
obtain rw access to the cib

Expected results:
must not be possible with read-only access to the cib to assign a role

Additional info: