Improve CentOS related installation docs

[obscurerichard]

I fixed up the CentOS related installation instructions to reflect all the changes needed, and to explain some of the tradeoffs offered to configure this module on CentOS or another Fedora derived system that runs SELinux.

This is tested vs. CentOS Linux release 7.7.1908 (Core) and icingaweb2-2.7.3-1.el7.icinga.noarch.

[obscurerichard]

@lippserd & @Al2Klimov any chance of getting a PR review and merge of this?

[cla-bot[bot]]

Thank you for your pull request. Before we can look at it, you'll need to sign a Contributor License Agreement (CLA).

Please follow instructions at https://icinga.com/company/contributor-agreement to sign the CLA.

After that, please reply here with a comment and we'll verify.

Contributors that have not signed yet: @obscurerichard

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Please contact us if you think this is the case.

• If you signed the CLA as a corporation, your GitHub username may not have been submitted to us. Please reach out to the responsible person in your organization.

[obscurerichard]

I've signed the CLA.

[bobapple]

@cla-bot check