Add `OpenStackServer` ref into `OpenStackMachine`

[EmilienM]

/kind feature

Describe the solution you'd like
When an OpenStackMachine is created and reconciled, we don't put its OpenStackServer ref in its status or anywhere else. We could add it so the link is easily discoverable (beside the fact the resources have the same name).

[lentzi90]

I am ok with dropping this. We should ensure that all relevant information is bubbled up to the OpenStackMachine instead.

[EmilienM]

I initially worked on a PR but it got no traction so far, happy to revisit later.