Add support for Proximity Placement Groups

[russcam]

Proximity placement groups achieve co-location of Azure Infrastructure as a Service (IaaS) resources with low network latency. See https://azure.microsoft.com/en-au/blog/introducing-proximity-placement-groups/ for more details.

The ARM template has historically used Availability Sets which were deployed in a single datacenter, but this assumption does not hold anymore. For low network latency, Proximity Placement groups should be used.