Retreive agent token from param store on windows agents

[chrisfowles]

fixes #761

Gets agent token from param store if BuildkiteAgentTokenParameterStorePath is set and BuildkiteAgentToken is not for windows instances.

[yob]

Thanks @chrisfowles 😍 Missing this on the windows side was definitely an oversight, and confirmed on a test stack based on this PR that the fix works.

My only thought is that current behaviour on linux is: use the SSM path if it's provided, overriding any manually provided token

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

Lines 107 to 109 in 857e3f7

	if [[ -n "${BUILDKITE_AGENT_TOKEN_PATH}" ]] ; then
	BUILDKITE_AGENT_TOKEN="$(aws ssm get-parameter --name "${BUILDKITE_AGENT_TOKEN_PATH}" --with-decryption --query Parameter.Value --output text)"
	fi

The difference is small, but maybe it's worth using the same behaviour as linux to keep the two code paths as similar as we can?

[chrisfowles]

Cheers! I've updated the logic to match the linux install @yob.

There's possibly some discussion around if this is the "correct" logic, as I'd typically expect an explicitly defined value to override a retreived value for application configuration. However it's probably not worth a breaking change on the linux side for just this qualm ;)

[yob]

lovely, thanks again.

I agree - I'd probably lean towards your initial behavior if we were starting from scratch, but I think the downsides are small enough that consistency with the (only recently) released behaviour is worth it.

I might backport this to a new 5-0-stable branch as well, and if no other significant bugs show up over the next few days then release a 5.0.1 stack.

[yob]

We've released this in 5.0.1. Thanks again!