Terraform config fixes based on testing (#207)

Terraform config fixes based on testing (#207)

* Terraform config fixes based on testing

* 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.
* Fix step numbering in QUICKSTART
* Remove sentence fragment about creating config from step 4 of
  QUICKSTART
* Add note about needing the specified ssh key to be loaded in your
  ssh-agent for provisioning to succeed