task/pexec: Output stderr to teuthology log
task/pexec: Output stderr to teuthology log

Signed-off-by: Sam Lang <70ab3b98ad075c66e7d3501cb42b80a7e41da316@inktank.com>

robbat2/teuthology,ceph/teuthology,robbat2/teuthology,dmick/teuthology,zhouyuan/teuthology,ceph/teuthology,tchaikov/teuthology,SUSE/teuthology,michaelsevilla/teuthology,zhouyuan/teuthology,dreamhost/teuthology,dreamhost/teuthology,michaelsevilla/teuthology,dmick/teuthology,t-miyamae/teuthology,yghannam/teuthology,SUSE/teuthology,ivotron/teuthology,t-miyamae/teuthology,dmick/teuthology,caibo2014/teuthology,SUSE/teuthology,ktdreyer/teuthology,tchaikov/teuthology,caibo2014/teuthology,yghannam/teuthology,ktdreyer/teuthology,ivotron/teuthology