Description of problem:
The agent's config export command produces an invalid XML file.

Bug 536138 introduced a change that results the DOCTYPE element being removed and no DTD being defined. This means that a config export from the agent produces a file that can not be imported back into the agent:

Failed to start the agent
java.lang.IllegalArgumentException: Failed to load configuration file [/tmp/00989956-agent-configuration-export.xml]. Cause: java.util.prefs.InvalidPreferencesFormatException: org.xml.sax.SAXParseException; lineNumber: 2; columnNumber: 41; Document root element "preferences", must match DOCTYPE root "null".
	at org.rhq.enterprise.agent.AgentMain.processArguments(AgentMain.java:3155)
	at org.rhq.enterprise.agent.AgentMain.<init>(AgentMain.java:508)
	at org.rhq.enterprise.agent.AgentMain.main(AgentMain.java:404)


Version-Release number of selected component (if applicable):
3.1.2.GA

How reproducible:
Always

Steps to Reproduce:
1.  Install and configure JBoss ON agent.
2.  Launch agent command prompt:

        ${RHQ_AGENT_HOME}/bin/rhq-agent.sh -n
        
3.  Execute agent config export prompt command:

        config export /tmp/exported-agent-configuration.xml
        
4.  Exit agent command prompt.
5.  Re-configure agent using export configuration:

        ${RHQ_AGENT_HOME}/bin/rhq-agent.sh --cleanconfig --config /tmp/exported-agent-configuration.xml

6.  Validate /tmp/exported-agent-configuration.xml:

        xmllint --valid /tmp/exported-agent-configuration.xml


Actual results:
Agent fails to start:

    Failed to start the agent
    java.lang.IllegalArgumentException: Failed to load configuration file [/tmp/00989956-agent-configuration-export.xml]. Cause: java.util.prefs.InvalidPreferencesFormatException: org.xml.sax.SAXParseException; lineNumber: 2; columnNumber: 41; Document root element "preferences", must match DOCTYPE root "null".
        at org.rhq.enterprise.agent.AgentMain.processArguments(AgentMain.java:3155)
        at org.rhq.enterprise.agent.AgentMain.<init>(AgentMain.java:508)
        at org.rhq.enterprise.agent.AgentMain.main(AgentMain.java:404)


xmllint reports:

    /tmp/exported-agent-configuration.xml:2: validity error : Validation failed: no DTD found !
    <preferences EXTERNAL_XML_VERSION="1.0">
                                           ^


Expected results:
Exported agent configuration file can be imported without error or validation warning.

Additional info:
As indicated in Bug 782915, this issue is a direct result of the work done in Bug 536138.