LU-3561 tests: improve sanity-hsm.sh to support remote agent

LU-3561 tests: improve sanity-hsm.sh to support remote agent

In the current sanity-hsm.sh, only one HSM copytool is started on
the local Lustre client node.

In order to test multiple remote HSM agents/copytools, this patch
introduces a new facet type named "AGT" and the following associated
variables:

- AGTCOUNT: number of agents
- AGTDEV{N}: target HSM mount point (root path of the backend)
- agt{N}_HOST: hostname of the agent agt{N}
- SINGLEAGT: facet of the single agent

By default, only single copytool is started on a remote client node.
If there was no remote client, then the copytool will be started on
the local client.

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I6e85d6948da894bd50148b0504347159ae23a79b
Reviewed-on: http://review.whamcloud.com/7110
Tested-by: Hudson
Reviewed-by: jacques-Charles Lafoucriere <jacques-charles.lafoucriere@cea.fr>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>