LU-11673 tests: quote argument of -n conf-sanity

LU-11673 tests: quote argument of -n conf-sanity

Inside the single bracket test function '[', the argument
of the ‘-n’ flag should be quoted arguments.  If the -n
argument is not quoted, a blank value will cause the
variable to disappear and this causes issues.  Quote the
argument or use [[ ]].

conf-sanity test 79 has two cases where the ‘-n’ argument
is not quoted.  Let's correct this.

Lustre-change: https://review.whamcloud.com/35669
Lustre-commit: 443cc6e51f0202b9bc40c256259c4fc14ae3f7af

Test-Parameters: trivial envdefinitions=ONLY=79 testlist=conf-sanity
Signed-off-by: James Nunez <jnunez@whamcloud.com>
Change-Id: I4b3a43de064d1992439dc25ecc7b0682520f74c9
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Wei Liu <sarah@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36928
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>