ipatests: add test_server_del to the nightly def
    
    Add the test to the nightly test suite with xfail annotation.
    
    Because of https://pagure.io/389-ds-base/issue/49972, the test
    may fail while re-installing the replica1. The issue exists only
    on fedora 27 with 389-ds 1.3.9.0-1.fc27 and not in RHEL 7.x as the fix
    is delivered in 389-ds 1.3.9.1.
    
    When test_ignore_topology_disconnect_replica1 fails, the 2nd next test
    test_removal_of_master_disconnects_both_topologies is also likely to fail.
    Note that the failure is intermittent, hence the xfail annotation does
    not use strict=True.
    
    Reviewed-By: Sergey Orlov <sorlov@redhat.com>