qa/tasks/ceph: resurrect log compression

qa/tasks/ceph: resurrect log compression

Commit 953662555840 ("qa/tasks/ceph: use Cluster.sh() and Remote.sh()
when appropriate") dropped run.wait(), which waits for all given
processes to exit.  This resulted in errors like

  INFO:teuthology.orchestra.run.smithi107.stderr:tar: ./objectstore_tool..log: file changed as we read it
  INFO:teuthology.orchestra.run.smithi107.stderr:tar: ./ceph-client.admin.175125.log: File removed before we read it

as the task moved on to archiving semi-corrupted and uncompressed logs,
filling up the lab cluster.

Revert that hunk, as Cluster.sh() is useless here -- we don't need
stdout or stderr, but very much need parallel execution and wait for
the compression to finish.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>