"preferred" export path not updated in the CephFS/NFS driver

[Goutham Pacha Ravi]

Description
===========
As part of a fix for https://launchpad.net/bugs/2035137 ; the CephFS/NFS driver was improved to return preferred export paths. This is useful when transitioning from standalone NFS-Ganesha server to cephadm deployed NFS-Ganesha cluster. While multiple export paths are represented, there are a couple of issues with how this fix is working

Steps to reproduce
==================

A chronological list of steps which will help reproduce the issue I hit:
* Start with a devstack with cephadm/nfs
* Set "cephfs_ganesha_server_ip" to the same IP as the VIP assigned to the cephadm deployed nfs cluster

Expected result
===============
when "cephfs_ganesha_server_ip" is set to the same value as the IP address of the new NFS cluster, i expect the driver to ignore the IP duplication. This is clearly a misconfiguration; but not critical enough to fail startup.
it is expected that the export path through the cephadm deployed nfs cluster has its "preferred" metadata set to True

Actual result
=============
- I have redundant export paths, because the driver isn't deduplicating export server IPs
- exports pertaining to the nfs cluster aren't set as "preferred=True"

Environment
===========
1. trunk branch of manila; but this fix has been backported to antelope (2023.1)

2. Ceph/NFS with Ceph Reef

[Vida Haririan (vhariria)]

See additional triage information at https://meetings.opendev.org/meetings/manila/2024/manila.2024-02-15-15.00.log.html