Fix JMA agent property names: use underscores not hyphens

[stokpop]

Problem

The Java Memory Assistant agent fails to start with:

com.sap.jma.Agent#ERROR: JavaMemoryAssistant cannot start
java.util.NoSuchElementException: The option 'jma.check-interval' is unknown

When using:

JBP_CONFIG_JAVA_MEMORY_ASSISTANT: '{enabled : true, agent: { thresholds : { heap: "80%" }, heap_dump_folder: /home/vcap/, check_interval: 5m } }'

Root Cause

The Go buildpack emits JVM system properties with hyphens, but the JMA agent expects underscores (matching the original Ruby buildpack):

Go (wrong)	Should be
-Djma.check-interval	-Djma.check_interval
-Djma.max-frequency	-Djma.max_frequency
-Djma.heap-dump-folder	-Djma.heap_dump_folder
-Djma.log-level	-Djma.log_level
-Djma.threshold.*	-Djma.thresholds.*

Additionally, heap_dump_folder set via config is silently ignored (always defaults to $PWD).