[validation] Fix the slave-label

Change the plugin used for docker build jobs. The jobs weren't
scheduled properly when using the slave-label parameter.

Also reduce the size of the slave for validation-{stream}-docker
job as we don't need docker to be on the slave

Change-Id: Ied06773d28fb8706a6072e744aa3c64cfba602f7
Signed-off-by: Cristina Pauna <cristina.pauna@enea.com>