UI should add button to download curtin-logs.tar on deployment failure

[Lee Trager]

When a deployment fails while Curtin is running Curtin uploads an additional file, curtin-logs.tar, which contains additional information about the failure. The MAAS and Curtin teams often need this file to debug bugs reported by issues. Currently this file can only be accessed over the API as follows

maas $PROFILE node-script-result download $SYSTEM_ID current-installation filters='/tmp/curtin-logs.tar' > curtin-logs.tar

Users often have difficultly using the API to get this file. It would be helpful for the UI to add a button to download curtin-logs.tar when available on the logs tab.

Its important to note that curtin-logs.tar is only available when a problem is detected while Curtin is running. Its possible for a deployment to fail outside of when Curtin is running meaning curtin-logs.tar won't be available. The button should only appear if curtin-logs.tar is actually available.