leave FD unassigned in the launch instance call

[QiangZhangO]

What would you like to be added:
it looks CAPOCI set a random number for FD before launch a new instance https://github.com/oracle/cluster-api-provider-oci/blob/main/cloud/scope/machine.go#L203-L212. We'd like to know if it is feasible to leave FD unassigned

Why is this needed:
in PCA/C3 env, the number of compute nodes might be uneven across Fault Domain. It might happen that some FD is full, while there are resources on the other two FDs. Compute team suggests to launch instance without specifying the fault domain so that compute service will distribute VMs based on availability across all FDs