spec: don't let parallel workers stomp each others' cassandras
spec: don't let parallel workers stomp each others' cassandras

at some point we started loading/caching the cassandra config before
forking, which made every worker share the same keyspaces, causing
various specs to fail during the initial run

test plan:
* the build should be more stable

Change-Id: I804178152c165d72f3665ce1103c8af4736a9aa6
Reviewed-on: https://gerrit.instructure.com/122252
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Tested-by: Jenkins
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
Product-Review: Jon Jensen <44f878afe53efc66b76772bd845eb65944ed8232@instructure.com>
QA-Review: Jon Jensen <44f878afe53efc66b76772bd845eb65944ed8232@instructure.com>

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