Option to keep a minimum instance count when doing rolling updates.

When doing a rolling upgrade operators should have an option to ensure the instance count doesn't go bellow the value stipulated. This would be accomplished by bringing the next instances up before putting down old ones.
Today the only behavior is so that if my instance count is 3 and I do a rolling update with max_parallel=1, for a moment I'll be left with only 1 instance ready.

Start by tracing Nomad's rolling upgrade behavior and how max_parallel controls instance replacement. Define how a minimum instance count should be preserved while bringing up replacement instances, then verify that a three-instance update with max_parallel=1 never drops below the stipulated ready count.

Written by the indexing model from the issue text.