Allow PULSAR_MEM & PULSAR_GC to be Overridden in pulsar_tool_env.sh (#…

…15868)

The pulsar_tool_env.sh sets the PULSAR_MEM and PULSAR_GC environment variables without allowing them to be overridden.
This change keps the default values but allows PULSAR_MEM & PULSAR_GC to be overridden which aligns with the
pulsar_env.sh file. This allows adjustments to be made to the memory settings when more memory is needed.

Co-authored-by: Jim Baugh <jim.baugh@oracle.com>

### Motivation
The pulsar_tool_env.sh sets the PULSAR_MEM environment variable without allowing it to be overridden. When running an pulsar-admin function (e.g. running the kafka to pulsar connector) we can hit java memory issues without a way to change the memory settings. This PR resolves this issue.

### Modifications
This change keeps the default value but allows PULSAR_MEM to be overridden which aligns with the
pulsar_env.sh file. This allows adjustments to be made to the memory settings when more memory is needed.

### Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

### Does this pull request potentially affect one of the following parts:

*If `yes` was chosen, please highlight the changes*

  - Dependencies (does it add or upgrade a dependency): (no)
  - The public API: (no)
  - The schema: (no)
  - The default values of configurations: (no)
  - The wire protocol: (no)
  - The rest endpoints: (no)
  - The admin cli options: (no)
  - Anything that affects deployment: (no)

### Documentation

Check the box below or label this PR directly.

Need to update docs? 
- [X] `doc-not-needed` 
(Please explain why)
There is not currently documentation around the pulsar_tools_env.sh PULSAR_MEM setting. This change doesn't change the default behavior.

conf/pulsar_tools_env.sh

@@ -42,10 +42,10 @@
 # PULSAR_GLOBAL_ZK_CONF=
 
 # Extra options to be passed to the jvm
-PULSAR_MEM="-Xmx128m -XX:MaxDirectMemorySize=128m"
+PULSAR_MEM=${PULSAR_MEM:-"-Xmx128m -XX:MaxDirectMemorySize=128m"}
 
 # Garbage collection options
-PULSAR_GC=" -client "
+PULSAR_GC=${PULSAR_GC:-" -client "}
 
 # Extra options to be passed to the jvm
 PULSAR_EXTRA_OPTS="${PULSAR_MEM} ${PULSAR_GC} ${PULSAR_GC_LOG} -Dio.netty.leakDetectionLevel=disabled ${PULSAR_EXTRA_OPTS}"