CentOS/RHEL 8 support

[arm4b]

Similar to recently added Ubuntu Bionic packages for StackStorm, add support for CentOS/RHEL 8 with python 3 and MongoDB 4.

[arm4b]

Initial EL8 support in StackStorm is finished 🏁 by @punkrokk and will be released with the st2 v3.2

[arm4b]

From the https://github.com/orgs/StackStorm/projects/9 project only post-tasks related to deployment methods (Ansible, Puppet, Chef, etc) are missing which will be added after v3.2 release.