Add debug logging to contextutil.nested.
Add debug logging to contextutil.nested.

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