Set $GALAXY_MEMORY_GB and allow for lowering values by an overhead.

[natefoo]

Fixes #15952.

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. Using Slurm you can verify that $GALAXY_MEMORY_GB is set properly using metrics or inspecting the tool_script.sh
  2. Using the local runner you can set $GALAXY_MEMORY_MB as an env var and the others should be set accordingly.
  3. Test the overhead and floor vars the same way, this simple tool is helpful:

     <tool id="vartest" name="vartest" version="0.0.1" profile="16.04">
         <command><![CDATA[
             echo "$echo" >  "$output1"
             ; declare -p GALAXY_MEMORY_MB >> "$output1" 2>&1 || true
             ; declare -p GALAXY_MEMORY_MB_PER_SLOT >> "$output1" 2>&1 || true
             ; declare -p GALAXY_MEMORY_GB >> "$output1" 2>&1 || true
             ; declare -p GALAXY_MEMORY_GB_PER_SLOT >> "$output1" 2>&1 || true
             ; declare -p GALAXY_MEMORY_MB_OVERHEAD >> "$output1" 2>&1 || true
             ; declare -p GALAXY_MEMORY_MB_FLOOR >> "$output1" 2>&1 || true
         ]]></command>
         <inputs>
             <param name="echo" type="text" title="Text to echo"/>
         </inputs>
         <outputs>
             <data name="output1" format="txt" />
         </outputs>
     </tool>

License

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