LU-2471 tests: Fix facet_type() to recognise names like mds1_2

LU-2471 tests: Fix facet_type() to recognise names like mds1_2

In conf-sanity 24a, mdsdevname() and mdsvdevname() are given facet
numbers like "1_2".  This causes facet_type() to be given facet names
like "mds1_2", which it can not return the facet type "MDS".  This
patch fixes facet_type() to handle names like "mds1_2" properly.

Signed-off-by: Li Wei <wei.g.li@intel.com>
Change-Id: I0203add5f6fdf089d5061ee48c6efca449db1a96
Reviewed-on: http://review.whamcloud.com/4849
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Wei Liu <wei3.liu@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>