Rename VanillaHealthIndicator to Application...

Rename `VanillaHealthIndicator` to `ApplicationHealthIndicator` and
changed the exposed bean name from `statusHealthIndicator` to
`applicationHealthIndicator`.

This provides less confusing JSON output:

  {"status":"UP","application":{"status":"UP"}}

vs:

  {"status":"UP","status":{"status":"UP"}}

Fixes gh-1291
Polish