Lower rax-ord max-servers in nodepool to 56

* .../templates/nodepool/nodepool.yaml.erb: While the max instances
limit in rax-ord is 100, the RAM quota is only enough to run 62 of
the 8G flavor. Drop that by enough to build images while running at
full steam, along with a safety margin of a couple more.

Change-Id: Ibc7afbd164e33320ca3a4b8e75de1f507d15153a
Merge "Lower rax-ord max-servers in nodepool to 56"