Move verify_release_build.sh to Jenkins job

verify_release_build.sh is used for validation after release branch is cut. Basically it does two things: 1. verify Gradle build with -PisRelease turned on. 2. create a PR and run all PostCommit jobs against release branch. However, release manager got many painpoints when running this script:

1. A lot of environment setup and some of tooling install easily broke the script.
2. Running Gradle build locally too extremely long time.
3. Auto-pr-creation (use hub) doesn't work.

We can move Gradle build to Jenkins in order to get rid of environment setup work.