LU-1607 tests: allow MODOPTS_LIBCFS module params

LU-1607 tests: allow MODOPTS_LIBCFS module params

The changes made for forcing non-default CPT partition counts in
commit 95d67b28076f7938a6c962a5256e9b581a439f71 now prevents
overriding libcfs module options using MODOPTS_LIBCFS, because
there is always the cpu_npartitions parameter specified at module
loading time that overrides the environment variable.

Instead add the cpu_npartitions=2 argument to the environment
variable instead of making it a parameter.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I94bf500164dc23e81080bd2675646b5fa7500c1e
Reviewed-on: http://review.whamcloud.com/3999
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>