Centralize yang-maven-plugin version

There is no reason for individual projects to define the version over
and over again -- let's keep it in one place.

Change-Id: I4ea6ccbc51e31cf3cef3b7c1304ea7dcc02667f7
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Centralize yang-maven-plugin version"