This adds the support for creating and deleting worker nodegroups
using different stack per nodegroup. In order to be backwards
compatible, default nodegroups will remain in one stack.
Having this in mind cluster status is now calculated aggregating the
statuses of the underlying stacks.
Change-Id: I97839ab8495ed5d860785dff1f6e3cc59b6a9ff7
(cherry picked from commit e52f77b299)