Support Lmod

I have rasied with AWS Support and they asked me to raise a Feature Request...

Please make sure to add the following data in order to facilitate the root cause detection.

**Required Info:**
- AWS ParallelCluster version [e.g. 3.1.1]: 3.7.2
- Slurm

**Bug description and how to reproduce:**
I would like the option to use Lmod instead of environment-modules

At present parallel cluster installs environment-modules and the current validator requires that implementation (see my PR https://github.com/aws/aws-parallelcluster/pull/3862)

I'd like to either
- have a parallel cluster option to choose which modules implementation to install
or failing that,
- be able to replace environment-modules with Lmod in my parallel cluster image

at present, due to the validator, I have to do the replacement in the user-data of the compute instances which adds to the, already too long, time it takes from a instance starting to it being available to the cluster and jobs starting on it

Thanks, Neil