torture: Rename RCU_QEMU_CMD to TORTURE_QEMU_CMD

This commit makes the torture scripts a bit more RCU-independent.
It also removes a redundant export of this same shell variable.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>