Deduplicate MapNode key leaf values

When we are encountering a leaf node inside a MapEntryNode, it can
be a leaf corresponding to a key -- in which case its value is
already present in NodeIdentifierWithPredicates and we do want to
de-duplicate those objects.

JIRA: CONTROLLER-1908
Change-Id: I2ed65c311f9921aa77c9f23bd1f7681d7f11355a
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>