Add option to filter build with matching when counting current running jobs

Note: Job in the same category but with no parameters are not counted either

Signed-off-by: Nicolas Morey-Chaisemartin <nmorey@kalray.eu>
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In our current CI at Kalray, we often have very long integration queues (build are very long and we have 30 people committing a lot).
To accelerate that, we want to run several integration (same project) in parallel. But it is only safe if these run do not target the same integration branch. 
The branch being one of the parameters, I added a setting to the throttle plugin so we don't count the overll number of task but only the task whom parameter (specified in the throttle config) match. 

This doesn't apply directly on 1.8.1 as I've been told it's buggy and is not written to be applied straight away.
If the feature seems interesting to other, it can be probably cleaned up and contributed. If not, I'm open to feedback on better ways to do this.