LU-12387 tests: Validate l_tunedisk max_sectors_kb tuning

Add test to ensure that l_tunedisk only tunes the max_sectors_kb
value of OST devices, and that it properly tunes any slave devices.

Test-parameters: trivial
Test-parameters: fstype=ldiskfs testlist=conf-sanity \
		 envdefinitions=ONLY=125
Cray-bug-id: LUS-7358
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: I414526e71fd7ac2811d7c0e8a6afd80a50788258
Reviewed-on: https://review.whamcloud.com/35081
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Gu Zheng <gzheng@ddn.com>
Reviewed-by: Nathaniel Clark <nclark@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>