Improvements for IMSM_NO_PLATFORM testing.

Improvements for IMSM_NO_PLATFORM testing.

Factor out IMSM_NO_PLATFORM testing into a single function that caches
the result.

Allow mdmon to explicitly set the result to "1" so that we don't need
the ENV var in the unit file

Check if the kernel command line contains "mdadm.imsm.test=1" and in
that case assert NO_PLATFORM.  This simplifies testing in a virtual
machine.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Jes Sorensen <jes@trained-monkey.org>