Environment module system now optional, but enabled by default

[jovial]

The use case is when you want to use a different module system to the one we supply or want to run without one entirely.

[sjpb]

Overal idea and interface seems fine, approach seems a bit nasty just to deal with omitting one package though as the version-specific vars are already a bit complex.

I suggest just using a conditional in (both) vars files to add an empty string if openhpc_module_system_install is false, then just filtering out empty strings in the yum install step. That's still easily extendable if we want to turn off other packages.