RFR 8023093: Add ManagementAgent.status diagnostic command

[Jaroslav Bachorik]

Please, review the following change

Issue : https://bugs.openjdk.java.net/browse/JDK-8023093
Webrev: http://cr.openjdk.java.net/~jbachorik/8023093/webrev.00 
(hotspot, jdk)

This change adds a new 'ManagementAgent.status' diagnostic command. This 
command will output the current status of the management agent in the 
following format

```
Agent: <enabled|disabled>

(
  ConnectionType: <local|remote>
  Protocol: <rmi|...>
  Host: <IP or host name>
  URL: <valid JMX connector URL>

  (
    Properties:
    (
      <propertyname>=<propertyvalue>
    )+
  )?
)+
```

Where:
  <name>    means an arbitrary value
  |         means 'or'
  ( and )   denote a block
  +         block repeats one or more times
  ?         block appears at most once

A new test is added exercising this diagnostic command. While adding the 
test I factored out the shared functionality from the JMXStartStopTest 
class and caused the changes in the test area to be a bit more extensive 
than just adding a new test class.

Thanks,

-JB-