LU-14627 tests: Create unload_modules_local

LU-14627 tests: Create unload_modules_local

t-f allows for loading modules on single node via load_modules_local.
However, there is no corresponding unload_modules_local that can be
called to cleanup after call to load_modules_local, so we create it.
unload_modules() refactored to use unload_modules_local.

Also address a potential issue that can prevent LND modules from
unloading. Some LNet setup (particularly those in sanity-lnet) may
require that we call lnetctl lnet unconfigure (or lctl net down)
to drop a ref on the module before it can be unloaded.

HPE-bug-id: LUS-9031
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: I6458a7728f5f559f8641c5a9e29dd775c8445c38
Reviewed-on: https://review.whamcloud.com/43425
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>