[FLINK-11786][travis] Merge cron branches into master

[zentol]

What is the purpose of the change

This PR merges all daily cron branches into the master. This significantly improves their visibility, makes them easier to maintain as well as simplify the process of running e2e tests for contributors.

cron-master-maven_compat will continue to exist since we only want to run this weekly and we can't have 2 separate cron intervals for the same branch.

Brief change log

• rework stage selection to be reliant on script parameters instead of implicitly relying on the order of jobs
• configure existing jobs to only be run for each pr/commit, excluding cron builds
• merge cron-master-hadoop24
• merge cron-master-scala212
• merge cron-master-docs
  • requires additional caching of .rvm so we don't re-download dependencies on every build
  • caching of other builds is not affected
• merge cron-master-e2e
• merge cron-master-jdk9
  • modify stage.sh to support excluding modules for which tests currently fail
  • modify travis_mvn_watchdog.sh to not run e2e tests (since they currently fail)

I made no attempts at de-duplicating code in various scripts (like compile calls). Ain't nobody got time for that.

Verifying this change

Clone my branch and setup a cron build. Note that a full build can take a few hours.
I have started a build for this PR: https://travis-ci.org/zentol/flink/builds/502561735

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[aljoscha]

This looks good to merge. The travis.yml is a bit bloated now but I guess that's what we have to do for now. I'm wondering what's up with the failure in flink-docs, here.

[zentol]

The PR is probably based on a broken master, rebasing and re-running travis to be sure.