Add MinProtocolVersion to autopilot config

[ChrisSchinnerl]

The goal is to allow people to define a cutoff for hosts below a certain version. e.g. v1.6.0 would mean that hosts with version v1.5.9 or below would get a 0 score.

• Add optional MinProtocolVersion setting to autopilot config #1143
• Add to UI

[alexfreska]

For the UI it's probably best to check if the autopilot is configured first. If it isn't, we can default to 1.6.0 unless the user sets a different value. If it is, we set the value to the empty string to not enforce a limit beyond the one the backend already enforces implicitly.

The "suggested" value we display to the user should probably be 1.6.0 too. To indicate that it's in the user's best interest to not use older hosts.

Regardless of the value set by the user, the backend won't use anything older than 1.5.9 but we can't implicitly have the backend set this value since it might cause data loss in the current landscape of hosts.