Issue an info message when a config=true list does not have keys

Issue an info message when a config=true list does not have keys

RFC7950 section 7.8.2 places an explicit requirement that lists
representing configuration (i.e. config=true) must define at least
one key.

This restriction does make sense in contexts like NETCONF, but is
generally not needed in OpenDaylight. Since it hurts model
interoperability, add a message pointing this violation out.

JIRA: YANGTOOLS-957
Change-Id: If29078e51aa1ec55fa070f263755f15147ad835f
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>