make initial server run during build optional via ENV var

[vorburger]

The currently commented out "initial server run during build" in .s2i/bin/assemble WOULD be handy to have it start up faster every time, and save e.g. re-downloading https://libraries.minecraft.net/net/minecraft/launchwrapper/1.12/launchwrapper-1.12.jar and creating the initial /config/ directory structure etc. etc. again and again.

Given the problems hit with it in some OpenShift deployments, let's perhaps have it built optionally via an ENV var (which is false for DISABLED by default) - and then experiment if by tweaking the YAML of Builders it can't be made to work anyway, if desired.