provider/azure: use affinity group in vnet site

Revert change based on observations of vnet
related issues. Azure console displays advisory
warning about vnets tied to locations, suggesting
vnets be tied to affinity groups to mitigate.
Merge pull request #204 from axw/azure-network-affinitygroup-1.20

provider/azure: use affinity group in vnet site

Revert change based on observations of vnet
related issues. Azure console displays advisory
warning about vnets tied to locations, suggesting
vnets be tied to affinity groups to mitigate.