Add optional MinProtocolVersion setting to autopilot config

[ChrisSchinnerl]

That allows users to define their own minimum protocol version. e.g. users might want to only use 1.5.9+ hosts so they can enforce that even though renterds minimum is set to 1.5.9.

If no version is set, only the global MinProtocolVersion is applied.

Related to #1141

[ChrisSchinnerl]

@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.