Add ability to override JSR-352 baseContext.xml

While existing functionality provided the ability to override the
components provided in the baseContext.xml (as required for JSR-352
functionality) at a job by job level, this commit allows a user to
specify a base context for all of the JSR-352 based jobs within a JVM.
To set the location of the custom base context, a JVM property
JSR-352-BASE-CONTEXT should be set.

BATCH-2290