Fixing issue where trend graph won't show if there is no build with an active workspace

This issue happens when JMeter is run in a Jenkins slave in EC2 and all slaves are pulled down.
Also updating the code so that it will not just pick the latest build directory, but will make
sure that the PerformanceReport directory is present. This will stop the trend graph disappearing
while the job is running.
Merge branch 'performance-1.8-OmnifonePatch'

Fixing issue where trend graph won't show if there is no build with an active workspace