Allow admin to override $GALAXY_MEMORY_MB

[natefoo]

Otherwise, $GALAXY_MEMORY_MB is unconditionally overridden.

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[natefoo]

Draft until I test on Main

[natefoo]

Works on Main.

[mvdbeek]

I suppose you'll also want to sync that back to pulsar (https://github.com/galaxyproject/pulsar/blob/master/pulsar/managers/util/job_script/MEMORY_STATEMENT.sh)?