Add devstack.exercise subtask
Add devstack.exercise subtask

The subtask runs devstack/exercise.sh

Signed-off-by: Zack Cerza <f801c831581d4150a2793939287636221d62131e@inktank.com>

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