fixes stderr handling in run_cqlsh
fixes stderr handling in run_cqlsh

Before, p.stderr was consumed printing each line. Now, stdout and stderr
are flattened to strings before being processed and/or returned.

kishkaru/ccm,mambocab/ccm,mike-tr-adamson/ccm,tolbertam/ccm,mike-tr-adamson/ccm,mikefero/ccm,tolbertam/ccm,pcmanus/ccm,thobbs/ccm,aboudreault/ccm,pombredanne/ccm,bcantoni/ccm,jorgebay/ccm,unusedPhD/ccm,umitunal/ccm,mikefero/ccm,slivne/ccm,mike-tr-adamson/ccm,pcmanus/ccm,AtwooTM/ccm,pcmanus/ccm,josh-mckenzie/ccm,thobbs/ccm,jeffjirsa/ccm,bcantoni/ccm