Fix bug in Ipfix table and TearDown IT code

The cache columns weren't added until schema version 7.3.0
This commit adds splits the IPFIX tests with Junit assumptions and as
such OVS 1.11.0 IT's will no longer fail

TearDown tries to delete a FlowSampleCollectorSet row which will not
have been created unless the schema version is greater than 7.1.0. This
commit conditionally adds this transaction in the teardown code.

Change-Id: I1dc5b13329f0f1f5bcee60625737871719d77df7
Signed-off-by: Dave Tucker <djt@redhat.com>