Prepare for an RPM Package

This merge request prepares for the creation of an slurm-singularity-exec.spec RPM package file:

• Use /usr/lib and /usr/libexec for installation
• Remove spank.h and include it from the environment, in order to match the host Slurm versions
• Adjust Vagrantfile for an Enterprise Linux 8 (EL8) box (aka. AlmaLinux 8)
• Adds slurm-singularity-exec.spec to build an RPM package