Parallel tests: run suites sequentially and make sure to have at least 1 concurrent run (Fixes #5091).
Parallel tests: run suites sequentially and make sure to have at least 1 concurrent run (Fixes #5091).

Summary:
Parallel tests together with ParallelTestExecution overwhelms the system.
Test fail, since not enough threads can be created.

On machines with one processor, the concurrency level needs to be set to 1.

Test Plan: sbt test

Reviewers: jeschkies, unterstein, jenkins

Reviewed By: jeschkies, unterstein, jenkins

Subscribers: jasongilanfarr, marathon-team

Differential Revision: https://phabricator.mesosphere.com/D482

janisz/marathon,gsantovena/marathon,mesosphere/marathon,natemurthy/marathon,guenter/marathon,guenter/marathon,guenter/marathon,meln1k/marathon,janisz/marathon,Caerostris/marathon,natemurthy/marathon,gsantovena/marathon,meln1k/marathon,guenter/marathon,meln1k/marathon,mesosphere/marathon,gsantovena/marathon,mesosphere/marathon,gsantovena/marathon,mesosphere/marathon,meln1k/marathon,natemurthy/marathon,gsantovena/marathon,Caerostris/marathon,janisz/marathon,Caerostris/marathon,Caerostris/marathon,janisz/marathon,janisz/marathon,meln1k/marathon,guenter/marathon,Caerostris/marathon,mesosphere/marathon