This patch fixes 2 issues in slack notification.
First, incoming webhook supports the JSON request, however Slack
API to post a message to chat doesn't support JSON request.
Change to use "data" for posting a message to chat.
Second, incoming webhook returns the response as text.
Change to check the content type of response before parsing it.
Change-Id: Ic81cc1d99496774a16f91569022d5e4fe0aff795
Story: 2001041
Task: 4597
4cfcf0803f