CBQE-995: fix for multi-workload  phases

When running multiple workloads in single phase
automation would advance to next phase before
all workloads complete.  This fix makes sure
all workloads with postconditionas are met before
moving to next phase

Change-Id: Icc5403f4ec294f2ddcd6a1448f4358e0d7205399
Reviewed-on: http://review.couchbase.org/24332
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>