Fix default value of host_subset_size in nova-controller config.

  The default value of host_subset_size has been set to 1.
'host_subset_size' is mostly for cases then multiple VMs booting simultaneously
 to avoid race conditions. If no such cases batter to use default value - 1

Closes-bug: PROD-36098
Change-Id: Id05658a17ba1e206009b9cfe8227359febce4377
7 files changed