[FEATURE] Support multiple OSes and architectures in same cluster

[cartalla]

Is your feature request related to a problem? Please describe.

ParallelCluster currently only supports 1 OS and 1 CPU architecture in a cluster.
Users should be able to define compute nodes that use any supported OS and architecture in the same cluster.

Describe the solution you'd like
In the InstanceConfig allow the specification of multiple OSes and architectures like in the legacy version.

Unlike the legacy version, it would probably be good to allow the instance types and counts to be separately specified for each OS/architecture permutation.
Maybe should also allow different custom AMIs as a key.