Install singularity by default in the compute node image

[chryswoods]

I think it would be worth making it easier for less technical users to install their software. I recommend that singularity be installed by default into the compute node images (and possibly into the mgmt node). While this could be done by adding the below to the compute_image_extra.sh file, it is an extra technical step that is too daunting for newcomers.

sudo yum install -y epel-release
sudo yum update -y
sudo yum install -y singularity

We may also want to consider installing EasyBuild and/or Spack by default too, so that it is easier for new users to type simple commands to install their software, without having to deal with rebuilding images.