Add nuxeo shell to all nuxeo distributions

We need to add the nuxeo-shell to be part of all nuxeo distributions - to have it working as an applet or as part of the launcher.
The JAR must be added in a directory named "client" in nxserver root. And the JAR should be the one build by nuxeo-shell-distribution (an all-in-one signed JAR)
Example: ${tomcat_distirb_root}/nxserver/client/nuxeo-shell-xxx.jar

Also we must use the public Nuxeo signature for production builds. Currently we are signing the JAR using a dummy (i.e. for development) certificate stored in the project sources. This is done by default (in the default "dev" profile).
On build servers we must add a certificate file to be used to sign the production all-in-one JAR. We can use a maven profile for this which will be activated if this file exists - otherwise the "dev" profile will be used (which is using the dummy signature)