Fix tentacle hostname resolution on non-domain-joined machines

[wlthomson]

Background

A customer running the Windows Tentacle container in ECS on EC2 reported that their Tentacle was being registered in
Octopus with an invalid URL. They had configured PublicHostNameConfiguration=FQDN so the container would register
using its internal EC2 DNS hostname (e.g. ip-xxx-xx-x-x.ec2.internal). This seems to be the right configuration when the Octopus server and the ECS task are in the same VPC, so should be supported.

Fixes HPY-1292

Results

The configure-tentacle.ps1 script constructs the FQDN by concatenating DNSHostName and Domain. On a domain-joined machine this is correct (DNSHostName is the short name and Domain is the DNS suffix). However, on a non-domain-joined machine like an EC2 instance, DNSHostName is already fully qualified and Domain is empty.

This PR adds a conditional to check if DNSHostName already contains a dot or Domain is empty before attempting to join them.

How to review this PR

• Main risk here is regressions. The conditional is intentionally restrictive to reduce the risk of changing desired existing behaviour, but the risk is still there.

Quality ✔️

• Unit testing this area is difficult, and adding an integration test for this use-case feels a bit disproportionate.
• If there are concerns, I can do some manual testing against a real EC2 instance (but again, time cost is high vs. the value of this change).

Pre-requisites

• I have read How we use GitHub Issues for help deciding when and where it's appropriate to make an issue.
• I have considered informing or consulting the right people, according to the ownership map.
• I have considered appropriate testing for my change.

[hnrkndrssn]

Do we also need to make sure the Linux equivalent script functions correctly?

Currently it executes hostname --fqdn to get the FQDN host name.

[wlthomson]

From my understanding, linux should be fine as hostname returns the complete FQDN. This problem arises from having to manually construct it on Windows from concatenation of two separate fields (from Win32_ComputerSystem).