Populate the Node Cache at the controller start/restart.

Populate the Node Cache at the controller start/restart.

This cache can be removed fully, but if we do that, we need
to make read call to the data store, where we will have to
read the whole topology tree, that can put some load on the
md-sal data store, and it might not be negligible in case of
clustered data store.

Change-Id: Ieeb5ae5ba01839b34aefe7d3916c6bdd0fb14808
Signed-off-by: Anil Vishnoi <vishnoianil@gmail.com>