Persisting a list element to a parent list (ep2)

Persisting a list element to a parent list (ep2)

Post List Element does not allow for create List Element, only appends onto existing node as children

-Add a check in saveListElements to see if the parent xpath is a root path ("/").If root node store list element as top node. Else add passed list element to parent xpath node.
-Add test for scenario for above
-Add test scenario Saving list element data fragment under Root node
-Add Integration Tests Add and Delete top-level list (element) data nodes with root node
-Update bookstore model with TopLevelList datanode

Issue-ID: CPS-1586
Change-Id: Iaa7f59fbeebb03703626132c6d5c2afde0e7ab4b
Signed-off-by: Rudrangi Anupriya <ra00745022@techmahindra.com>