LU-8275 tests: add flag to enable secret shared key for tests

LU-8275 tests: add flag to enable secret shared key for tests

When the SHARED_KEY environment variable is set to true,
test-framework will set up a shared key between the nodes and start
Lustre with shared key enabled. Three tests (28,29,30) are also
added to sanity-sec in order to test shared key features.

Signed-off-by: Kit Westneat <kit.westneat@gmail.com>
Signed-off-by: Chris Hanna <hannac@iu.edu>
Change-Id: I2abff35fbb7dd4f227c6cd12a66cb0b77f610157
Reviewed-on: https://review.whamcloud.com/20780
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>