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?

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[obscurerichard]

I've signed the CLA.

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