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.
Add link to docs
Merge pull request #197 from axw/azure-network-affinitygroup

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.