Fix min/max validation inside written value

Fix min/max validation inside written value

When the user is writing a complete tree min/max elements
validation needs to occur inside validateStructure(), as that
runs when the data is being introduced. If we fail to validate
it then, the parent will be committed with the assumption that
the written value actually conforms to the schema.

Fix MinMaxElementsValidation to check the number of elements
inside verifyStructure(), so we flat out reject invalid writes.

JIRA: YANGTOOLS-776
Change-Id: Ie804463612403c578f0f9230884ae53641ba76c6
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>