Supports selectively overriding net.ipv4.ip_forward sysctl var

[conorsch]

The README previously documented a non-existent default var called os_network_forwarding. This commit adds it back, with documentation. The goal is to support selectively overriding only a single sysctl var, that which permits IPv4 forwarding. Presumably this is the most commonly overridden sysctl var for this role—see #50 for discussion.

This commit simply adds sugar to the outstanding #67. @fitz123, feel free to chime in if this is useful for you too.

[rndmh3ro]

I purposefully left that out, because now you have a variable that sets another variable, which is kind of redundant.
Then one could ask the question why only the ipv4.ip_forward variable gets a special treatment and not the others. (See #50, the option "use something like this" on why not to do this)

The way it currently is you have documentation for every variable next to it and can change it as desired, without another abstraction.

[fitz123]

I'm doubt) I need overwrite ipv4.ip_forward, but since sysctl dict is in defaults, it's easier to have own dict with all variables you need and from time to time (when ansible hardening framework updated) compare what's new in there. for example, forwarding in my specific case is needed only for firewalls/routers/vpn servers and they're kind of special guys comparing with usual serves (so anyway need changes to sysctl)

otherwise you will just end up with separate variable for each sysctl dict value, and it already has been discussed a lot and community conclusion is keep it as a dict.

[conorsch]

Roger that, let's stick with #67 to solve the problem.