Enable IMDSv2 in AWS Windows stemcell

When the bosh-linux-stemcell-builder was updated to add IMDSv2 support
to AWS Linux stemcells in commit
86bb00e0b64ea7ece71ed2775358fbab99cef033 (see:
<cloudfoundry/bosh-linux-stemcell-builder@86bb00e>),
stembuild was not updated with the corresponding change.

This commit makes that change. It looks like without this change, Bosh
Agents running in Windows on IMDSv2-only VMs will be unable to talk to
the AWS Metadata Server, and will be unable to start. JIRA ticket
BOSHWIN-43 was opened with a customer complaining of this very problem.

Commentary from the bosh-linux-stemcell-builder commit mentioned above
follows:

- TokenPath must be set in `agent.json` in order for the bosh-agent to load metadata using the IMDSv2 protocol.
- This commit adds that field. It will be ignored unless the agent has been updated to expect it.

[#180052419] [TAS-94] Convert the Agent on AWS to use IMDSv2

modules/BOSH.Agent/BOSH.Agent.psm1

@@ -91,6 +91,7 @@ function Write-AgentConfig {
                 "URI" = "http://169.254.169.254"
                 "UserDataPath" = "/latest/user-data/"
                 "InstanceIDPath" = "/latest/meta-data/instance-id/"
+                "TokenPath" = "/latest/api/token"
             })
             "UseRegistry" = $true
           }