Terraform config fixes based on testing

[brianloss]

• Update AzureRM Terraform provider to use any 3.x version.
• Fix bug where variable validation would fail for the Azure configuration if managed_disk_configuration was not used and set to null.
• Clarify in the README that terraform init/plan/apply commands are sensitive to the current working directory.
• Clarify in the README that the appropriate SSH private key must be present in the SSH agent in order for terraform apply to succeed.