Updates to UI Remote build feature

[richard-cox]

• Default ui-offline-preferred to Local (note the actual value might not be that string)
  • Change in rancher/rancher repo
• Remove the defaults for ui-index and ui-dashboard-index
  • Change in rancher/rancher repo
  • Update references in our docs to contain the values that were removed
  • Add info to our docs on how to supply them, and change ui-offline-preffered, as helm and docker cli args
  • Announce change to extended team
• Add a disclaimer shown when setting ui-index or ui-dashboard-index
  • Above input, only when editing or modal?
  • TBD but something like 'Use of Rancher Artifacts requires acceptance of the SUSE EULA'

The above changes, in particular how it affects -head builds, should be announced to the wider team. This should contain instructions on how to install rancher with the previous env (for example if QA need to test setup process changes)