roachprod: rewrite roachprod start

This is quite the workhorse, and does a lot and has to be compatible
with a lot of existing CRDB versions. It's grown organically as a result
and I'm finding it a bit difficult to maintain, breaking it down a bit
makes it clearer what the structure of it all is and would've perhaps
prevented me introducing bugs like cockroachdb#51497.

Do scrutinize the PR closely, we use `roachprod start` everywhere. It's
mostly mindless code movement but I did sneak in the fix for cockroachdb#51497
where I'd broken node ID assignments for when `roachprod start` is
called with the `--sequential` flag (true by default). I did so by
explicitly initializing the first node, and then having the remaining
nodes join on to it.

Release note: None

pkg/cmd/roachprod/install/cluster_synced.go

@@ -102,7 +102,13 @@ func (c *SyncedCluster) IsLocal() bool {
 	return c.Name == config.Local
 }
 
-// ServerNodes TODO(peter): document
+// ServerNodes is the fully expanded, ordered list of nodes that any given
+// roachprod command is intending to target.
+//
+//  $ roachprod create local -n 4
+//  $ roachprod start local          # [1, 2, 3, 4]
+//  $ roachprod start local:2-4      # [2, 3, 4]
+//  $ roachprod start local:2,1,4    # [1, 2, 4]
 func (c *SyncedCluster) ServerNodes() []int {
 	return append([]int{}, c.Nodes...)
 }