task.ssh_keys(): manage authorized_keys, not keys2
task.ssh_keys(): manage authorized_keys, not keys2

See the issue for a description, but authorized_keys2 is dead
on RHEL7 and dying elsewhere.

Fixes: #9812
Signed-off-by: Dan Mick <b07550071eaa6a9296289c43bbd6c90559196431@inktank.com>

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