Set default minimum count in scaling policy to 0

Previously the minimum was not set, thus defaulting to the value of count.
This did not allow creating execution environments with a prewarmingPoolSize
of 0 as the task group count must not be less than the minimum coun in the
scaling policy.

environment/default-job.hcl

@@ -13,6 +13,7 @@ job "default-poseidon-job" {
     count = 1
     scaling {
       enabled = true
+      min = 0
       max = 300
     }
     spread {