LU-13160 tests: fix sanity-hsm monitor setup

On RHEL8, even we are using pdsh -R ssh,
the ssh still waits for the remote cat process
to finish.
Use the subshell to avoid the time out.

Change-Id: Id5b8d492b5ce9a235da73448ade475ade145bbed
Test-Parameters: trivial clientdistro=el8.1 testlist=sanity-hsm
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-on: https://review.whamcloud.com/37595
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>