backend plugins: fix crashes in development mode
    
    Do not set or delete attributes directly on KerberosWSGIExecutioner, ldap2
    and ra_lightweight_ca instances, as that raises an AttributeError in
    development mode because of ReadOnly locking.
    
    Use the usual workaround of `object.__setattr__` and `object.__delattr__`
    to fix the issue.
    
    https://pagure.io/freeipa/issue/6625
    
    Reviewed-By: Tomas Krizek <tkrizek@redhat.com>