LU-10638 build: add support for Scientific Linux

LU-10638 build: add support for Scientific Linux

Add Scientific Linux to Distro with a reference to RHEL.

    https://en.wikipedia.org/wiki/Scientific_Linux

Note that development of Scientific Linux has been discontinued,
but is still receiving LTS maintenance for EL6- and EL7-based
releases, so should not be removed until all support for those
distros has been removed.

Signed-off-by: Alexander Boyko <c17825@cray.com>
Change-Id: I05001ee9233e4cac6b2c6b523a374ffdc243394d
Cray-bug-id: MRP-3882
Reviewed-by: Artem Blagodarenko <c17828@cray.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>