kola/tests/*: always use 'c' for 'TestCluster'
kola/tests/*: always use 'c' for 'TestCluster'

It turns out rkt tests were the only ones not already conforming with
that convention.

Having all tests conform with it will make 'sed' refactoring easier.

glevand/coreos--mantle,dm0-/mantle,dm0-/mantle,glevand/coreos--mantle