admin_client.py: Assign and Remove role to user points to a different URI

Description:

The URI's in the methods "assign_user_role" and "remove_user_role" are different from that that appear when same action is done using "Kaystone Client".

Changing this URI would result in resolving Bug #999219 and Bug #1022990

Hence 3 Test cases would PASS that are SKIPPED now.

Existing URI:
1. assign role to user on tenant:
'users/%s/roleRefs' % user_id, post_body, self.headers

2. remove role from user
'users/%s/roleRefs/%s' % (user_id, role_id)

New URI:
1. assign role to user on tenant:
'/tenants/%s/users/%s/roles/OS-KSADM/%s'
                                % (tenant_id, user_id, role_id), post_body,
                                   self.headers

2. remove role from user
('/tenants/%s/users/%s/roles/OS-KSADM/%s'
               % (tenant_id, user_id, role_id))