LU-11607 tests: add a default definition for SINGLEMDS

LU-11607 tests: add a default definition for SINGLEMDS

Move the $SINGLEMDS definition from the test config file into
get_lustre_env() so that it is always set.  It should use facet
"mds1", anything else probably doesn't work.

Test-Parameters: trivial
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Iecbd6fe7d5102da7c2c14741c85986cf73054035
Reviewed-on: https://review.whamcloud.com/36093
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Wei Liu <sarah@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>