Improving the format for service-configuration.xml

Now, there is no need to comment a service definition when you want to
disable a service.

To disable a service, just add enabled="false" as an attribute to the
service tag and that's it, for example:

<service enabled="false">
    <name>OpenNMS:Name=Capsd</name>
    <class-name>org.opennms.netmgt.capsd.jmx.Capsd</class-name>
    <invoke at="start" pass="0" method="init"/>
    <invoke at="start" pass="1" method="start"/>
    <invoke at="status" pass="0" method="status"/>
    <invoke at="stop" pass="0" method="stop"/>
</service>

The default value is 'true'. If the attribute is not specified, it is
assumed to be 'true'.

The current content of the file has been updated to use this new
attribute for the services that are going to be disabled by default.
Fix for NMS-6970 - Upgrade Needed for service-configuration.xml

The upgrade task is going to parse the current existing file, and it is
going to override it with the fixed version using the new convention for
disabling services.