Adding support for Commbench as a new experiment, also adding El Dorado as a system

[simongdg]

Adding El Dorado system (docs: Adding a System)

• Add systems/snl-eldorado/system.py file - only if it is different from El Capitan software stack
• Should be able to use El Capitan's systems/all_hardware_descriptions/hardware_name/hardware_description.yaml which will appear in the docs catalogue
• Update system.py to use Spack 1.0 @scheibelp

Adding Commbench benchmark (docs: Adding a Benchmark)

• package.py
• update package.py to use Spack 1.0 @scheibelp
• application.py
• Tags in Ramble's application.py or in repo/benchmark_name/application.py will appear in the docs catalogue
• Add experiment.py to define a single node and multi-node experiments

[scheibelp]

This appears similar to https://github.com/LLNL/benchpark/blob/develop/systems/llnl-elcapitan/system.py, so I added some references about what needs to change with the merge of #953

(package recipe looks compatible, no notes there)

[scheibelp]

The specification for this changed: it looks like https://github.com/LLNL/benchpark/blob/develop/systems/llnl-elcapitan/system.py is a close match (uses all the same compilers). If you have questions about that, let me know.

[simongdg]

I have updated my pull request so that the snl-eldorado system looks more similar to llnl-elcapitan system. I also undid the cray-mpich package.py to how it was originally, as I was able to add the gtl_lib_path to my snl-eldorado system.py.