Installation docs should stress requirement for CLEAN system for one-line install

[LindsayHill]

The installation docs https://docs.stackstorm.com/install/ say "Get yourself a clean 64-bit Linux box that fits the system requirements."

For a one-line install, it assumes a clean, basic system. But people try to run the installer on customized systems that have things like non-standard authentication and $HOME setups, and this breaks. It is possible to install ST2 on those systems, but it needs to be a manual install, not a one-line install.

We have added checks to the one-line install script to look for things like disk space, ports in use, etc. That helps, but we can't catch everything.

We need to update the docs to stress this more clearly - a one-line install only works on an unadulterated Linux system. Perhaps a warning box, or similar?

[LindsayHill]

Resolved with #433