[UI/Papercut] Deploy: Selecting "Register as MAAS KVM host" overrides user OS selection without confirmation

[Edin S]

UI/Papercut bug

Note: it can be argued this isn't a bug -- but I certainly encountered it as a first-time user of MAAS.

MAAS host OS: Ubuntu 18.04.3 (x86_64) MAAS version: 2.6.0 (7802-g59416a869-0ubuntu1~18.04.1)

How to reproduce: Given a machine is in a "Ready" state (i.e. commissioned and ready to be deployed)
1. Navigate to the machine's "Deploy" page (e.g. /MAAS/#/machine/XXXX -> Take action -> Deploy)
2. For "Choose your image", select the following options:
   - Ubuntu
   - 19.04 "Disco Dingo"
   - Default kernel
   For "Additional installs" select:
   - (enable) Register as MAAS KVM host
3. Click the "Deploy machine" button

Bug: the release 19.04 was selected, but selecting "Register as MAAS KVM host" will force the host OS installed to be 18.04LTS -- without notifying the user.

Expected behaviour: Prompt the user that "Register as MAAS KVM host" requires the deployed OS to be 18.04LTS (or other LTS).