refactor: simplify GetSubnet function

[dkoshkin]

What problem does this PR solve?:
LMK if I'm missing something but I didn't see why this function couldn't resemble

cluster-api-ipam-provider-nutanix/internal/client/cluster.go

Lines 40 to 47 in efd121a

	func (n *clusterClient) GetCluster(cluster string) (*Cluster, error) {
	clusterUUID, err := uuid.Parse(cluster)
	if err == nil {
	return n.getClusterByExtID(clusterUUID)
	}
	return n.getClusterByName(cluster)
	}

Which issue(s) this PR fixes:
Fixes #

How Has This Been Tested?:

Special notes for your reviewer:

[jimmidyson]

The logic here is due to the fact that a subnet name itself could actually be a uuid so we need to check first if it's the extID and then if no match there then check if there is one with that name.

We should actually update the get cluster logic to match this existing get subnet logic rather than the other way around.

[dkoshkin]

The logic here is due to the fact that a subnet name itself could actually be a uuid so we need to check first if it's the extID and then if no match there then check if there is one with that name.

IIUC you're saying thats this is handling the case where the user given name of the Subnet is a UUID but that name doesn't match the UUID of the external ID? Then it falls back to to finding the Subnet by name, a UUID in this case.

[jimmidyson]

@dkoshkin

IIUC you're saying thats this is handling the case where the user given name of the Subnet is a UUID but that name doesn't match the UUID of the external ID? Then it falls back to to finding the Subnet by name, a UUID in this case.

Correct - although this may sound odd, @thunderboltsid convinced me to support this in previous discussions.