LU-1538 tests: allow SLOW from environment for auster

LU-1538 tests: allow SLOW from environment for auster

Allow "SLOW=yes" to be passed from the environment when running
auster via acceptance-small.sh.  Otherwise, it overrides the value
passed from the environment.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I646adad77ade050c80e0da174dedb25d23500c1e
Reviewed-on: http://review.whamcloud.com/4707
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Li Wei <wei.g.li@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>