Add testcase for multidc scaleout with 90% utilization in parallel

[pehala]

At 90% storage usage, scale out each DC by adding a single node to each of them in parallel.

[cezarmoise]

Results

Without throttle r/w during scaleout

Initial Cluster: 2 DC with 3 x i4i.large each
Write to 70%; Sleep 30 min
Write to 90%; Sleep 30 min
Writes have RF=3
Add 1 x i4i.large to each DC
Sleep 30 minutes

With throttle r/w during scaleout

Initial Cluster: 2 DC with 3 x i4i.large each
Write to 70%; Sleep 30 min
Write to 90%; Sleep 30 min
Writes have RF=3
Add 1 x i4i.large to each DC
Sleep 30 minutes

[pehala]

Merged into #9157