rcutorture: Remove RCU dependencies from ver_functions.sh API

The current set of functions in ver_functions.sh have APIs that are
specific to RCU.  This commit therefore makes an RCU-independent function
that outputs version-specific boot arguments.  This has the benefit that
a test-type-independent call in kvm-test-1-rcu.sh can now handle any type
of test, given a test-type-specific set of files in a configs directory.

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