Allow configuring a list of subnet IDs for an AWS provider label instead
of just a single one. With this change we can avoid the need to create
multiple providers that only differ on the subnet. In addition we allow
the AWS fleet service to make better decisions when all possible subnets
are included.
For AWS fleet requests we will provide launcht template overrides for
each subnet. For normal run-instance requests - which only support
requests with a single subnet - we will select a random subnet id from
the list.
Change-Id: Ib6700c54920981271daad944a642741ed6187811
e64c559f47