Fix initialization value of stack_cost

Fix initialization value of stack_cost

After having recently fixed the job-cost script in
1c99557ff1b60356f85193d00e23020bb9e53467 I realized that I should check
over the stack_cost generation. Much to my surprise I found that the
cost was being initialized to 0.7! This means that all stack costs that
we've generated so far have been increased by $0.70 in the reported
cost, job runs that have no stacks are still properly reporting $0 for
the cost.

This change fixes the initialization to be properly 0.0.

Issue: RELENG-2719
Change-Id: I8474604dbe0e2b32165b086754e29fac17f79785
Signed-off-by: Andrew Grimberg <agrimberg@linuxfoundation.org>