add include element to params

Enable reading parameters from other xml files, e.g.:
<params>
<include file="my_params.xml"/>
</params>

Content of file my_params.xml:
<params>
<param name="foo" value="bar"/>
<param name="myParam" value="1"/>
</params>

Aditionally, parameters can be read from non-standard XML-Files by configuring an XPath expressions which points to the root of the param elements:
<params>
<include file="other_params.xml" node="/root/jobschedulerparams[@system="dev"]/params"/>
</params>

Content of file other_params.xml:
<root>
<jobschedulerparams system="dev">
<params>
<param name="foo" value="bar"/>
<param name="myParam" value="1"/>
</params>
</jobschedulerparams>
<jobschedulerparams system="prod">
<params>
<param name="foo" value="foo"/>
<param name="myParam" value="2"/>
</params>
</jobschedulerparams>
</root>