xfstest: add configurable load factors

xfstest: add configurable load factors

Most stress test has probable behavior, the longer test run the
larger corner cases will be cover. It is reasonable to allow
user to provide some sort of system load factor.
This patch introduce two global variables
LOAD_FACTOR: Usually means factor number of running tasks
TIME_FACTOR: Usually means factor of run time, or number of operations
If not specified both variables defined to 1, so original behavior
preserved.

TODO: Change all stress tests to use this variables

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>