Enable changelog in Ambry project

[mockitoguy]

Enable changelog in Ambry project so that their releases page will look great. Current Ambry releases: https://github.com/linkedin/ambry/releases

Also, it's going to be another customer.

[mockitoguy]

On a high level, we want the Ambry project to be configured similarly to https://github.com/shipkit/shipkit-demo project (shipkit-demo is a great reference on how to set it up).

More details (quickly written, I may have missed things ;-):

• apply the changelog plugin Ambry's main build script: https://github.com/linkedin/ambry/blob/master/build.gradle#L19, add the dependencies to buildscript: https://github.com/linkedin/ambry/blob/master/gradle/buildscript.gradle#L21
• work in Ambry's ci-release.gradle file.
  • configure the changelog tasks in, refer to shipkit-demo
  • posting releases to GH creates tags automatically, so we can get rid of gitTag, gitPush tasks and "./gradle/git-push.sh" file from Ambry
• bonus: update versions of shipkit plugins, remove unnecessary line: https://github.com/linkedin/ambry/blob/master/build.gradle#L20