limit the proofsrv slaves to be only two for these unit tests

[smuzaffar]

Default limit is 12 and during IB validation (which now runs on openstack machines) one of the proofserv.exe hanging. This should avoid these stable processes

[cmsbuild]

A new Pull Request was created by @smuzaffar (Malik Shahzad Muzaffar) for CMSSW_7_5_X.

limit the proofsrv slaves to be only two for these unit tests

It involves the following packages:

FWCore/TFWLiteSelector

@cmsbuild, @Dr15Jones can you please review it and eventually sign? Thanks.
@Martin-Grunewald, @wddgit, @wmtan this is something you requested to watch as well.
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@Degano you are the release manager for this.
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