LU-3142 tests: gather logs on passive server nodes

This patch adds all_mdts_nodes(), all_osts_nodes(), all_server_nodes()
and all_nodes() into the test-framework.sh to get the active and
passive server nodes. By using these functions, after recovery-*-scale
tests failed in failover configuration, the logs on passive server
nodes can also be gathered.

This patch also fixes yml_entities() to add MGS entity and entities
for passive server nodes.

Test-Parameters: clientdistro=el6 serverdistro=el6 clientcount=4 \
osscount=2 mdscount=2 austeroptions=-R failover=true \
useiscsi=true testlist=recovery-mds-scale

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: Idbba6f02f4f50b7d5097a78ea6b6954d5347919a
Reviewed-on: http://review.whamcloud.com/6112
Tested-by: Hudson
Reviewed-by: Li Wei <wei.g.li@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Hongchao Zhang <hongchao.zhang@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>