Enlist as KVM host overrides OS version

When Deploying a machine selecting 'Enlist as KVM host' silently overrides the selected os version.

How to reproduce:

1) Ensure you have downloaded ubuntu 20.04 via the images tab.

2) Select a machine to deploy.

3) Select ubuntu 20.04 from the version selection.

4) Select a non standard kernel from the drop down menu.

5) Select 'enlist as KVM host'

Perform deployment.

Deployment will fail, stating "not a supported kernel version for 18.04", indicating that the version of ubuntu has been overridden as a result of selecting 'Register as a KVM host'

Suggested fix.

On selection of 'Register as KVM host', a confirmation to the user that 18.04 will be used for the installation, rather than the selected 20.04

-- or --

Disable Register as KVM host when a kernel other than 20.04 is selected.