Add GetSubTree method for single interface

Perform a GetSubTree mapper call and return the mapper response
structure as is for a single interface over a given path subtree depth.

Tested:
    Returned subtree structure matches mapper structure for an interface
    Returned subtree data matches mapper data for an interface

Change-Id: If6b66433a331ac20633ac99c458eb5edb653bff2
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
1 file changed