cloud_aws: add multiple instance types to launch instance

AWS is running out of capacity for certain instance types (e.g. c3.large)
in most regions. Provide the capability to give a list of instance types
to workaround that problem
Merge branch 'multi_instance_types' of git://github.com/lamadon/poni

* 'multi_instance_types' of git://github.com/lamadon/poni:
  cloud_aws: add multiple instance types to launch instance