linter error fixes for ccmlib/cluster.py
linter error fixes for ccmlib/cluster.py

pcmanus/ccm,pcmanus/ccm,bcantoni/ccm,tolbertam/ccm,jorgebay/ccm,AtwooTM/ccm,unusedPhD/ccm,mike-tr-adamson/ccm,thobbs/ccm,mikefero/ccm,tolbertam/ccm,pombredanne/ccm,thobbs/ccm,mike-tr-adamson/ccm,pcmanus/ccm,mikefero/ccm,kishkaru/ccm,mike-tr-adamson/ccm,bcantoni/ccm