[do not merge] Per-OS installation instructions

Fixes various things like CentOS 7 SCL instructions, per-Debian/Ubuntu release
sources, more detailed instructions for RHEL channels and explicit Puppet Labs
repo instructions.

TODO before merging: tidy up default view, empty pre-reqs section, line-break issues

Opening in case anybody was interested (@mmoll perhaps).
nvm, opened domcleal/theforeman.org#1