LDAP: Use randomized ccname for storing credentials
    
    https://fedorahosted.org/sssd/ticket/2410
    
    If two ldap_child processes attempt to prime the ccache at the same time
    for the same domain, the ldap_child might fail with:
        [ldap_child_get_tgt_sync] (0x0040): Failed to init ccache: Internal credentials cache error
        [main] (0x0020): ldap_child_get_tgt_sync failed.
    
    To avoid the race-condition, the ldap_child process now creates the
    ccache randomized and before returning to the caller, renames the
    randomized ccache to a permanent one.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>