Issue with Azure Marketplace deployment (in existing VNET)

[vyunev]

Describe the bug
When Azure Marketplace Self-Managed solution is deploying in existing VNET, there is no option to put fully qualified subnet addresses/names as it required in the template ("Full reference to existing subnets"). Putting just subnet names is not supported in the template via next code:

if(equals(parameters('virtualNetworkNewOrExisting'), 'new'), resourceId(parameters('virtualNetworkResourceGroupName'), 'Microsoft.Network/virtualNetworks/subnets/', parameters('virtualNetworkName'), parameters('masterSubnetName')), parameters('existingMasterSubnetReference'))]"

To Reproduce
Try to create new deployment to existing network and put subnet name with full reference like

"/subscriptions/a245d7e9-021e-420c-a9fb-512ce55XXXXX/resourceGroups/RGNAME/providers/Microsoft.Network/virtualNetworks/VNETNAME/subnets/Infra".

It is not supported via field validation. If instead you put just subnet name like "Infra" deployment will fail with next error:

Property id 'existing' at path 'properties.ipConfigurations[0].properties.subnet.id' is invalid. Expect fully qualified resource Id that start with '/subscriptions/{subscriptionId}' or '/providers/{resourceProviderNamespace}/'.

Expected behavior
When deploying solution from Azure Marketplace it should support full reference for existing subnets for corresponding params. Or template should be updated to support just short names of existing subnets when we deploying in existing VNET.