Add a "skip" parameter

[GoogleCodeExporter]

With multi modules projects, you have to perform two commands to run devserver 
or update :
- mvn install on the parent to build the various modules
- mvn appengine:update on the ear module

It much faster to have only one command to run, and which would not require 
installing the other projects. One solution would be to add a "skip" parameter 
to the plugin so that we can add <skip>true</skip> to the non-ear modules.

Original issue reported on code.google.com by david.ha...@revevol.eu on 17 Nov 2013 at 2:51