App health

[seeker815]

The branch has the following changes

• [x] Created a new check for Marathon App HTTP Checks
• [x] Added the Check to list of Checks for each App

[ashwanthkumar]

@seeker815 Quick couple of feedback

• Please format the file using go fmt
• Please add tests
• Please don't use snake characters for variable names. Consider using camelCase like in all other files.