upgrade_test: increase schema error factor
upgrade_test: increase schema error factor

We have upgrade (debian9) tests failed for got 70+ schema load error,
it's still acceptable. Let's increase the error factor.

Now we allow shards_num * 3 schema load errors per host during the entire test.

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

scylladb/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-longevity-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests