Build RPMs correctly.

As discussed internally, probably the easiest way to install Easybuild
would be to build RPMs/DEBs and then `yum localinstall` them.

These changes allow to build RPMs, and ensure they depend on
`environment-modules`.  Strictly speaking, Bash, CMake and friends are
dependencies for specific EasyConfigs, not for the framework itself.

Somebody else may contribute the configuration for building DEBs.