[fix][slack] fix typo in slackNotification func

```
groovy.lang.MissingPropertyException: No such property: toLowerCase for class: java.lang.String

```
    us: https://mirantis.jira.com/browse/PRODX-3254

Change-Id: I80643deb85dcd61ade65e176c8832bd8370fb9bb
1 file changed