NEXUS-6735: Wait only if parallel build is used

Do not enter the loop to wait for other modules (needed in case of parallel build) when non-parallel build is underway. Hence, the endless loop that happens within wait will not happen. Also, this fixes the "extra 2sec" pause that entering the wait method introduces.

Issue
https://issues.sonatype.org/browse/NEXUS-6735

CI
http://bamboo.s/browse/NX-MVNF4