2995 check if storage network attached before cluster network and vlan config config deletion (backport #115)

[mergify]

Problem:
1.User can delete the cluster network when storage network NAD is still attached to it.
2.User can delete the vlan config when storage network NAD is still attached to it.

The above breaks the networking functionality of storage networks without user being notified.

Solution:
Do not delete the cluster network or vlan config if storage network still exists for the cluster.

Related Issue:

Test plan:
Manually validated in a harvester node.

Reference bug report:
harvester/tests#1311

Negative: Delete cluster network when already set on storage network

Navigate to Networks -> Cluster Networks/Configs, create Cluster Network named vlan2
Create Network Config vlan2-config for all nodes
Navigate to Advanced -> Settings, edit storage-network
Select Enable then select vlan2 as cluster network, fill in VLAN ID and IP Range
Navigate to Networks -> Cluster Networks/Configs, delete Cluster Network vlan2
Warning or error message should displayed

Negative: Delete vlan config when already set on storage network

Navigate to Networks -> Cluster Networks/Configs, create Cluster Network named vlan2
Create Network Config vlan2-config for all nodes
Navigate to Advanced -> Settings, edit storage-network
Select Enable then select vlan2 as cluster network, fill in VLAN ID and IP Range
Navigate to Networks -> Cluster Networks/Configs, delete network config vlan2-config
Warning or error message should displayed

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This is an automatic backport of pull request #115 done by [Mergify](https://mergify.com).