slurm-plugin: force apt update before install

Added Node.apt_command('update') calls before attempting to install
SLURM-related pacakges. Plugin fails without this fix due to old
version numbers being cached on the machine AMI; i.e., 'apt-get install'
would fail when attempting to install previous versions of the packages
that are no longer available in the Ubuntu apt repository.
Merge branch 'slurm-plugin' into develop