CLC-4256 Run load tests with both empty and primed cache

JIRA: https://jira.ets.berkeley.edu/jira/browse/CLC-4256

Reading our first 2-pass load test results at 
https://test.ets.berkeley.edu/tsung-app_calcentral/calcentral-dev/results/20140923-0703/report.html I realized that the empty-cache and primed-cache should be in separate test runs. Otherwise it's too hard to tell where the distinct phases begin and end.