Raise proper exception when LDAP limits are exceeded
    
    ldap2 plugin returns NotFound error for find_entries/get_entry
    queries when the server did not manage to return an entry
    due to time limits. This may be confusing for user when the
    entry he searches actually exists.
    
    This patch fixes the behavior in ldap2 plugin to
    1) Return even a zero search results + truncated bool set in
       ldap2.find_entries
    2) Raise LimitsExceeded in ldap2.get_entry and
       ldap2.find_entry_by_attr instead of NotFound error
    
    This changed several assumptions about ldap2.find_entries
    results. Several calls accross IPA code base had to be
    amended.
    
    https://fedorahosted.org/freeipa/ticket/2606