Subworkflow lib not found in classpath when parent workflow lib overwrites it

OOZIE-567 addresses the bug mentioned in the title here - by adding the subworkflow libpath to the classpath in addition to parent workflow libpath. OOZIE-871 reverted this to avoid duplicate values, but the subworkflow libpath should not be overwritten by parent workflow's libpath. Hence, tackling this bug here.

Existing testcase modified slightly to demonstrate the expected behavior.