ci: testplan: do not start more runners than needed

We have been launching 10 runners up a certain number of tests, although
we only neeed half of that for some scenarios.
Too many runners started that have to execute just a small number of
tests wastes times on setup and blocks the queue. Just start the number
of nodes needed based on initial calculation.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>