LU-5472 tests: export UMOUNT to avoid SLES12 issue

LU-5472 tests: export UMOUNT to avoid SLES12 issue

sles12 umount command has issue with '-d' option.
it will report error while a absolute pathname
of mountpoint present with '-d'. So we export
UMOUNT to avoid such problem. In fact, loopdev
can be free automaticly. So '-d' needn't given
explicity.  But consider compatibility we still
keep it.
Also include http://review.whamcloud.com/#/c/14799/
port patch to master.
In SLES12, umount command will run statfs() on the
filesystem, which will cause unmounting Lustre client
hang when OST is unavailable. This patch adds "-f"
option to zconf_umount in conf-sanity.sh to avoid the
issue.

Test-Parameters: alwaysuploadlogs envdefinitions=ONLY=32 clientdistro=sles12 mdtcount=1 testlist=sanity
Test-Parameters: alwaysuploadlogs envdefinitions=SLOW=yes clientdistro=sles12 mdtcount=1 testlist=conf-sanity
Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Change-Id: If466c2101e0db52b5ec1f7273a846dc2497cfb84
Reviewed-on: http://review.whamcloud.com/16445
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Jenkins
Reviewed-by: Jian Yu <jian.yu@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>