get agent token from param store

Author: chrisfowles

packer/windows/conf/bin/bk-install-elastic-stack.ps1

@@ -77,6 +77,12 @@ If ($Env:BUILDKITE_AGENT_ENABLE_GIT_MIRRORS_EXPERIMENT -eq "true") {
   $Env:BUILDKITE_AGENT_GIT_MIRRORS_PATH = "C:\buildkite-agent\git-mirrors"
 }
 
+# Get token from ssm param (if we have a path) if it's not set explicitly
+If (($null -eq $Env:BUILDKITE_AGENT_TOKEN -or $Env:BUILDKITE_AGENT_TOKEN -eq "") -and
+    ($null -ne $Env:BUILDKITE_AGENT_TOKEN_PATH -and $Env:BUILDKITE_AGENT_TOKEN_PATH -ne "")) {
+  $Env:BUILDKITE_AGENT_TOKEN = $(aws ssm get-parameter --name $Env:BUILDKITE_AGENT_TOKEN_PATH --with-decryption --output text --query Parameter.Value --region $Env:AWS_REGION)
+}
+
 $OFS=","
 Set-Content -Path C:\buildkite-agent\buildkite-agent.cfg -Value @"
 name="${Env:BUILDKITE_STACK_NAME}-${Env:INSTANCE_ID}-%n"