Work around vagrant-aws plugin bug

mitchellh/vagrant-aws#538 would block the
automatic download of AWS vagrant boxes unless we explicitly specify
an AMI, which in turn would then always overwrite the default from
the box.

Workaround as per the comment.

basebox/aws/linux/Vagrantfile

@@ -1,3 +1,6 @@
+$settings[:aws_boxes] ||= []
+$settings[:aws_boxes] << "tqtc/linux-aws"
+
 Vagrant.configure('2') do |config|
     config.vm.guest = :linux
     config.vm.provider :aws

basebox/aws/windows/Vagrantfile

@@ -1,3 +1,6 @@
+$settings[:aws_boxes] ||= []
+$settings[:aws_boxes] << "tqtc/windows-aws"
+
 Vagrant.configure('2') do |config|
     config.vm.guest = :windows
     config.vm.provider :aws

minicoin/lib/aws.rb

@@ -13,6 +13,8 @@
 # AWS specific settings
 def aws_setup(box, machine)
     $settings[:aws] ||= {}
+    $settings[:aws_boxes] ||= []
+
     return unless Vagrant.has_plugin?('vagrant-aws')
     return if $AWS_CLI_INSTALLED == false
     # this has to happen on machine level, even though it's only needed for the
@@ -27,10 +29,15 @@ def aws_setup(box, machine)
          # this group is created by minicoin with permissions for SSH, RDP, and WinRM
         aws.security_groups = [ "minicoin" ]
 
-        # if the box is not installed or doesn't specify an AMI, then the minicoin.yaml file
-        # has to specify it. And we can't override what is set here in the box's Vagrantfile,
-        # so we are stuck. https://github.com/mitchellh/vagrant-aws/issues/538
-        aws.ami = box.minicoin.machine['ami'] unless box.minicoin.machine['ami'].nil?
+        # Workaround for https://github.com/mitchellh/vagrant-aws/issues/538: if the box we
+        # want is not installed yet, then the AWS plugin fails the validation before the box
+        # gets downloaded and installed. To check whether the box is installed, we use an entry
+        # in our global settings hash that boxes add themselves to via their Vagrantfile.
+        # If the box is not loaded yet, then setting the ami to a dummy value satisfies the
+        # plugin without overwriting the box file or the AWS-specific provisioning declared
+        # in the minicoin machine configuration.
+        aws.ami = "dummy" unless $settings[:aws_boxes].include?(box.minicoin.machine['box'])
+
         aws.tags = {
             "minicoin" => box.minicoin.machine['name']
         }