LU-14844 tests: make sure mgc_requeue_timeout_min exist.

LU-14844 tests: make sure mgc_requeue_timeout_min exist.

The module parameter mgc_requeue_timeout_min was introduced to reduce
testing times. Currently the test framework always attempts to set this
value but it doesn't exist in earlier Lustre versions which breaks
interop testing. Set the module parameter only if it exist.

Change-Id: I64f62e3d6e2faeba99ced98363d241083f95d92e
Fixes: 04b2da6180d ("LU-14516 mgc: configurable wait-to-reprocess time")
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/44215
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>