Add property "yang.skip"

Add property "yang.skip"

This property allows the plugin to be configured so
to skip the generation of java sources out of yang files.
The purpose is to save the time spent genereting java code
that is already in place, and the user knows that haven't
changed since the last compilation.

Usage:
   mvn  install -Dyang.skip=true

Change-Id: I39d3da20864f7655cae68dfa9f4b67ec96137d9f
Signed-off-by: David <david.lopez.munoz@ericsson.com>