Allow pulsar_tool_env.sh PULSAR_MEM & PULSAR_GC to be Overridden

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.

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}"