Fix agent's signal-grace-period-seconds

We've added `signal-grace-period-second` to the CloudFormation in #1468, but it weren't referenced in the configuration file's template.

Fixes #1530

Author: scadu

packer/linux/conf/bin/bk-install-elastic-stack.sh

@@ -293,6 +293,7 @@ disconnect-after-idle-timeout=${BUILDKITE_SCALE_IN_IDLE_PERIOD}
 disconnect-after-job=${BUILDKITE_TERMINATE_INSTANCE_AFTER_JOB}
 tracing-backend=${BUILDKITE_AGENT_TRACING_BACKEND}
 cancel-grace-period=${BUILDKITE_AGENT_CANCEL_GRACE_PERIOD}
+signal-grace-period-seconds=${BUILDKITE_AGENT_SIGNAL_GRACE_PERIOD_SECONDS}
 signing-aws-kms-key=${BUILDKITE_AGENT_SIGNING_KMS_KEY}
 verification-failure-behavior=${BUILDKITE_AGENT_SIGNING_FAILURE_BEHAVIOR}
 EOF