[2.5, UI] Deploy KVM host - If the default deployment OS is non bionic+, clicking Register as MAAS KVM host doesn't set bionic

[Andres Rodriguez]

If the selected deployment OS is *not* bionic+, e.g. it is Xenial, clicking on 'Register as MAAS KVM host' doesn't automatically set bionic, but rather, it removes the selection and allows the deployment (which is tracked in (https://bugs.launchpad.net/maas/+bug/1804547)

1. Import Xenial/Bionic images
2. Select Xenial as the deployment OS
3. Click on 'Register MAAS KVM host' -> the OS selection gets reset to nothing instead of auto-selecting Bionic (see 02-clicking-removes-os.png )
4. It allows it to be deployed without setting the OS (see https://bugs.launchpad.net/maas/+bug/1804547)

The expected result is that when clicking 'Register MAAS KVM host' is clicked, 'bionic' is auto-selected as the latest available LTS.

PS. The information about LTS is already available in the commissioning OS< because the only supported commissioning OS are LTS.