thread_group needs to convert the count param to an integer

[AndyKriger]

If you try to use a property ${__P(thread_count)} to set a thread_group count parameter, you will get an errror ruby-jmeter/extend/threads/thread_group.rb:7:in `thread_group': undefined method `/' for "${__javascript(parseInt('${__P(thread_count)}'))}":String (NoMethodError)

extend/threads/thread_group.rb needs to use params[:count].to_i on line 6 so that the division on line 7 will work correctly