qa/tasks/ceph: create a log file before redirecting to it

qa/tasks/ceph: create a log file before redirecting to it

as it is shell who interprets ">>" and redirect the stderr to given
file, but the shell process is launched by ubuntu:ububunt without using
sudo, so the command fails with "Permission denied" failure. to address
this issue, in this change, a file with proper priviledges is created
beforehand using `install`, so shell is able to write to it.

also, instead of creating this file in `maybe_redirect_stderr()`, it
returns the command to create the log file.

Signed-off-by: Kefu Chai <kchai@redhat.com>