LU-2592 tests: do not remove $TMP/*active in rpc.sh

LU-2592 tests: do not remove $TMP/*active in rpc.sh

The $TMP/*active files record the current active facets info
under failover test configuration. They are removed from
init_test_env() initially. If failover tests use do_rpc_nodes()
which performs rpc.sh and in which init_test_env() is used,
then the active facets info will be lost during the testing.

This patch fixes the above issue by introducing an RPC_MODE
variable which controls that the $TMP/*active files will not
be removed from init_test_env() in rpc.sh.

Test-Parameters: envdefinitions=SLOW=yes,ENABLE_QUOTA=yes \
clientcount=4 osscount=2 mdscount=2 austeroptions=-R \
failover=true useiscsi=true testlist=replay-vbr

Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I7f23c58e06e76d23ee6be8f779218e8d8ff32ae1
Reviewed-on: http://review.whamcloud.com/5126
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Wei Liu <wei3.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>