contint: migrate slaves to use role pbuilder class

misc::package-builder is used on labs but would creates the cowbuilder
under /var/ which is too small on eqiad instances.  Instead use the role
class role::package::builder::labs which make sure additional space is
mounted at /mnt/ and get the cowbuilder images there.

Change-Id: Ice85f5695828780055df1758593f60c55b4bbc01