Multiple entry points to MiniAccumuloClusterImpl#_exec cause orphaned processes

There are presently multiple entry points to MiniAccumuloClusterImpl#_exec(Class<?> clazz, List<String> extraJvmOpts, String... args). #exec(Class<?> clazz, List<String> jvmArgs, String... args) and #_exec(Class<?> clazz, ServerType serverType, String... args).

Only in the former method do started processes get added to the cleanup list.

This causes obvious process leaks as can be observed in running MonitorSslIT where the Monitor is started in the test, but not stopped. MAC abandons it and does not know to stop it.