Variables expansion in the payload

### Jenkins and plugins versions report

In one of my jobs I have this with credentials:
```
credentialsBinding {
string('BOT_TOKEN', 'bot-token')
usernamePassword('OTHER_CREDENTIALS', 'other-credentials')
}
```

Then I have this with Generic Webhook Trigger:
```
genericVariable {
key("COMMENT_TEXT")
value("\$.comment.text")
}
```

If in the comment text I have something like:
```
Use the variable ${BOT_TOKEN} and ${OTHER_CREDENTIALS}.
```

Then inside of the shell command I have
```
echo "${COMMENT_TEXT}"
```

The output will be:
```
Use the variable **** and ****.
```

I have this job working as a webhook for Bitbucket, so when someone adds a comment to a PR a message is sent in Mattermost to the user. Then the person will receive not the `***`, they are receiving the actual credentials, but they were supposed to receive the literal `Use the variable ${BOT_TOKEN} and ${OTHER_CREDENTIALS}.`.

I believe the plugin is doing the expansion or I'm missing some configuration.