Restricted Jenkins builds on Ubuntu machines

[Martin Desruisseaux]

Hello all

Some time ago, we were having JVM crashes on the Solaris node of Jenkins 
server. Now, our builds on the Solaris nodes are failing since April 
20th because of insufficient memory space. Other projects have reported 
the same problem, and some peoples suggested that it could be caused by 
zombie processes. However given that the Solaris node seems to have more 
issues than the Ubuntu ones, I have restricted the Jenkin builds to the 
Ubuntu nodes for now.

    Martin