contint: monitor CI server diskstats in Ganglia

Reuse the diskstat plugin introduced previously, should provide us
diskstats metrics for the contint production slaves (gallium and
lanthanum).

The monitoring is added at the role level to make sure it will be
applied whenever we add a new Jenkins production CI slave.

Bug: 36994
Change-Id: Id0517440c6bdcbef01deca7ddb7327ce4d55123b