bump jmxfetch to 0.21.0 (#3783)

Author: arbll

config.py

@@ -43,7 +43,7 @@
 
 # CONSTANTS
 AGENT_VERSION = "5.28.0"
-JMX_VERSION = "0.20.2"
+JMX_VERSION = "0.21.0"
 DATADOG_CONF = "datadog.conf"
 UNIX_CONFIG_PATH = '/etc/dd-agent'
 MAC_CONFIG_PATH = '/opt/datadog-agent/etc'