Returns all datacenters in a vCenter.

    service_instance
        The Service Instance Object from which to obtain cluster.

    datacenter_names
        List of datacenter names to filter by. Default value is None.

    get_all_datacenters
        Flag specifying whether to retrieve all datacenters.
        Default value is None.

def get_datacenters(service_instance, datacenter_names=None,
                    get_all_datacenters=False):
    '''
    Returns all datacenters in a vCenter.

    service_instance
        The Service Instance Object from which to obtain cluster.

    datacenter_names
        List of datacenter names to filter by. Default value is None.

    get_all_datacenters
        Flag specifying whether to retrieve all datacenters.
        Default value is None.
    '''
    items = [i['object'] for i in
             get_mors_with_properties(service_instance,
                                      vim.Datacenter,
                                      property_list=['name'])
             if get_all_datacenters or
             (datacenter_names and i['name'] in datacenter_names)]
    return items