cql_test_env: Ignore error if user already exists
cql_test_env: Ignore error if user already exists

When a `cql_test_env` points to a data directory that was previously
populated with `cql_test_env`, then the "tester" user will already
exist. This is not an error, so we can just ignore the exception.

Fixes #3224.

Tests: unit (debug)
Signed-off-by: Jesse Haber-Kucharsky <526eadea4792054b99d5fa5dd6be9acdd76c5418@scylladb.com>
Message-Id: <7729e5a98d8020a7ed1b6d12d8726559f0850f9d.1519315698.git.526eadea4792054b99d5fa5dd6be9acdd76c5418@scylladb.com>

scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla