package-builder.pp: parameterized $pbuilder_root

The class define a $pbuilder_root which was not always used. Replace any
occurence of '/var/cache/pbuilder' with the $pbuilder_root variable.

Also, on labs /var is only a 2GB partition, we need to pass a different
base path for example /mnt/pbuilder which would be a LVM mount with much
more disk space.

Change-Id: Ie0fd7c9eef4dcdff826e523433fca0df7265cf2d
Merge "package-builder.pp: parameterized $pbuilder_root" into production