JMX_PORT is not used

Although JMX_PORT is assigned and JMX_MODE=local, (defaults) JMX server is not started

**To Reproduce**
Steps to reproduce the behavior:
1. Start IotDB cluster node with default settings.
2. Using bash, look if 31999 port is open
3. In logs, there is a phrase like
`[main] WARN o.a.i.d.s.StartupChecks:38 - iotdb.jmx.port missing from iotdb-env.sh(Unix or OS X, if you use Windows, check conf/iotdb-env.bat) `

**Expected behavior**
JMX server to started

**Kubernetes Environment**
- Apache IotDB 0.12.3

**Additional context**
I also tried to set JMX_PORT, JMX_MODE and iotdb.jmx.port as envrionment variables without success

Reproduce the default cluster-node startup and inspect the 31999 port and the StartupChecks warning about iotdb.jmx.port. Start by reading iotdb-env.sh and conf/iotdb-env.bat, then trace how JMX_PORT, JMX_MODE, and iotdb.jmx.port are read. Done means the default configuration starts the JMX server on the expected port without manual environment-variable changes.

Written by the indexing model from the issue text.