- Reduce image size by skipping weak package references.
- Consolidate and sort packages to install.
- Improve consistency between versions.
- Combine yum/dnf install and clean to avoid unwanted caching.
- Don't update existing packages from base image unnecessarily.
* Merge conflicts:
* [skip ci] Revert changes to run_tests.sh
gundalow will update this in a different PR
* [skip ci] Add in ubuntu1604 and opensuseleap
NOTE: We are not configuring anything to use these new images yet.
Therefore no impact on Travis performance
* python-mysql for opensuse
* It's mysql-server on centos6
