CiscoFCSanLookupService.get_device_mapping_from_network method should not use zoning_vsan as the key of the device mapping dictionary

In Cisco fc san lookup service implementation, source code from master branch below:

https://github.com/openstack/cinder/blob/master/cinder/zonemanager/drivers/cisco/cisco_fc_san_lookup_service.py

In CiscoFCSanLookupService class, its get_device_mapping_from_network method should not use zoning_vsan as the key of the returned device mapping dictionary.

Because this method looks up through all the san fabrics defined in Cinder configuration, and users may define multiple san fabrics
in the configuration, e.g., multiple principle switch IP addresses, so Cinder can manage multiple san fabrics, and for each principle switch, the zoning vsan can be the same. For example, users can define a vsan id 15 in switch 1 as well as switch 2. Let's assume WWN1 is in vsan 15 of switch 1, and WWN2 in vsan 15 of switch 2. Then the CiscoFCSanLookupService.get_device_mapping_from_network method(['WWN1', 'WWN2'], ['WWNx',...]), then this method will most likely return something like this (assuming vsan 15 of switch 1 is iterated first than vsan 15 of switch 2)
{
'15': {
'initiator_port_wwn_list': ('WWN2', '...')
 'target_port_wwn_list': ('WWNx', ..)
}
}

This is wrong. Instead, we should use fabric_name as the key, just as Brocade lookup service does.