provisiond: next is not incremented if workload id is unknown

[maxux]

When some reservation on the explorer contains a workload with an unknown type by provisiond, it get stuck on that id and doesn't fetch (next is not incremented) new items from the explorer, provisiond is stuck.

[maxux]

Not fixed. Issue is still present after pull request.

The error is generated by https://github.com/threefoldtech/zos/blob/master/pkg/provision/explorer/source.go#L39-L42

https://github.com/threefoldtech/tfexplorer/blob/master/client/workloads.go#L140-L141