Accept Custom Values for Network and Storage Pool in e2e test suite

Currently MicroCloud e2e test suite doesn't allow to customize the storage pool name or the network name used during the testing and defined in the `main.tf`. Its expecting `remote` pool and `default` network to be available.

This will work for most of the scenarios. But it would be nice to make it part of the `variables.tf` to allow testing on customized environments.

```
# Devices
device {
name = "root"
type = "disk"
properties = {
pool = "remote"
path = "/"
size = "4GiB"
}
}

device {
name = "eth0"
type = "nic"
properties = {
network = "default"
}
}
}
```

Start with the e2e Terraform configuration in main.tf and the existing inputs in variables.tf. Trace how the device pool and network values are currently fixed, then make those values configurable through the variables file. Run the e2e suite in a customized environment and confirm it no longer requires the remote pool or default network names.

Written by the indexing model from the issue text.