[BEAM-3255] Enabling gradle-based release process

[pabloem]

This creates a release task that emulates the workflow to create release candidates. The workflow for this is as follows:

# Create environment variables. Supposing that we're preparing the 2.5.0 release:
RELEASE=2.5.0
NEXT_VERSION=2.6.0
CANDIDATE=1
BRANCH=release-${RELEASE}

# Create a release branch
git branch ${BRANCH}

# Now change the version in existing gradle files, and Python files
sed -i 's/'$RELEASE'/'${NEXT_VERSION}'/g' build_rules.gradle
sed -i 's/'$RELEASE'/'${NEXT_VERSION}'/g' gradle.properties
sed -i 's/'$RELEASE'/'${NEXT_VERSION}'/g' sdks/python/apache_beam/version.py

# Save changes in master branch
git add gradle.properties build_rules.gradle sdks/python/apache_beam/version.py
git commit -m "Moving to ${NEXT_VERSION}-SNAPSHOT on master branch."

# RELEASE TASK
# In the release branch, run the release task
git checkout ${BRANCH}
./gradlew release -Prelease.newVersion=${RELEASE}-SNAPSHOT \
                  -Prelease.releaseVersion=${RELEASE}-RC${CANDIDATE} \
                  -Prelease.useAutomaticVersion=true --info --no-daemon

The release task must be performed each time a new Release Candidate is created, and once one is approved, then it's just necessary to add a new tag for the release:

git tag ${RELEASE} ${RELEASE}-RC${CANDIDATE}

The release task does the following:

• Checks that no commits / updates are needed to the code
• Checks that project has no SNAPSHOT dependencies
• Runs build
• Changes version in gradle.properties to remove -SNAPSHOT
• Performs a commit
• Adds a tag like so: v${RELEASE}-RC${CANDIDATE}
• Changes version in gradle.properties back to contain -SNAPSHOT
• Performs a commit

[pabloem]

This task is based on using the https://github.com/researchgate/gradle-release#introduction gradle plugin

[pabloem]

Retest this please.