Group Shell Scripts by Activity and not CI

[rnveach]

Re-identified at #11028 (comment) but discussed at other times. Making sure we have an issue.

It has become common that we have to rotate CIs when one becomes over bloated, one starts to have reliable issues, etc. all of which we have run into before.

We should group all our shell scripts by their activity instead of strictly putting it in the CI. For example, we should have script for regression, another for pitest (which we already do), etc...

Travis as one example still requires us to put commonds in shell. For the split we want, we should use a pattern similar to the below:

no-error-spring-cloud-gcp)
  mvn -e --no-transfer-progress clean install -Pno-validations
  ./.validation.sh no-error-spring-cloud-gcp

This way CI config and CI specific shell remove all CI specific commands, and under them there is group's shell script (validaiton.sh in this case) that try to be CI agnostic.