Add MET/METplus Variables to modulefiles

[EdwardSnyder-NOAA]

Is your feature request related to a problem? Please describe.
Recently, the SRW App changed the way it runs the verification tests by loading the MET/METplus modules versus declaring MET/METplus paths in the experiment. Because of this change, a few MET/METplus variables need to be added to their lua files in order for these tests to run.

Describe the solution you'd like
To resolve this issue, we would need to add these missing variables to their lua files. Please see the code block below for the requested changes needed in https://github.com/JCSDA/spack-stack/blob/release/1.4.1/configs/common/modules.yaml

met:
  environment:
     set:
       'MET_INSTALL_DIR': '{prefix}'
       'MET_BASE': '{prefix}/share/met'
metplus:
  environment:
     set:
       'METPLUS_PATH': '{prefix}'

Additional context
The upcoming SRW App release will be using the spack-stack-1.4.1 and will support the MET/METplus tests. Meaning we would like to add these changes to the spack-stack-1.4.1, if possible. If not, we can manually add these variables. But regardless we would like to have these changes in the develop branch before the next spack-stack release.

[climbfuji]

We had an extensive discussion on which variables are needed and should be set by spack-stack just a few weeks ago. It was decided that what is in spack-stack now is sufficient. But maybe that's no longer the case?

See #757

Adding @AlexanderRichert-NOAA and @ulmononian .

[EdwardSnyder-NOAA]

Thank you for bringing this previous discussion to my attention. I'll use the suggestion mentioned in it (exporting the *_ROOT variables) as a solution since we need this change in for the upcoming SRW App release.

The best I can tell is that these variables are used in running the METplus wrapper scripts. It is used by all of the MET packages. So if users aren't using the wrapper scripts there is no need for these variables. Therefore, I'd leave these variables out of their respective lua files for now.

[climbfuji]

Thanks @EdwardSnyder-NOAA ! The less customization we put into the spack-stack code the better. I'll close this issue.