mesh: Validate application by comparing composition data

mesh: Validate application by comparing composition data

Instead of validating application by enumerating D-Bus objects, create a
temporary node instance and check if composition data generated for the
temporary matches the node loaded from storage.

This allows node validation logic (primary element, mandatory models etc)
to be confined in node_generate_comp() function.

This also streamlines code implementing Attach(), Join() and
CreateNetwork() calls.

Change-Id: I82c634a32a89c446937a7d58ff4f9a73da339aa3
Signed-off-by: Anupam Roy <anupam.r@samsung.com>