transaction log uses QUORUM rather than LOCAL_QUORUM

This was reported on [Stack Overflow](https://stackoverflow.com/questions/48593532/janusgraph-nohostavailableexception-in-multi-datacenter) and [janusgraph-users](https://groups.google.com/forum/#!topic/janusgraph-users/lc9xr6fecBQ).

> We are using JanusGraph 0.2.0 with Cassandra 3.11.1 and testing its support for geo diversity. Currently we have 2 datacenters with 1 nodes in each and replication factor is 1 for both the datacenter.

**janusgraph-cassandra.properties**

```
storage.backend=cql
storage.cql.read-consistency-level=LOCAL_QUORUM
storage.cql.write-consistency-level=LOCAL_QUORUM
storage.cql.local-datacenter=dc2
storage.cql.only-use-local-consistency-for-system-operations=true
storage.cql.replication-strategy-options=dc1,1,dc2,1
storage.cql.replication-strategy-class = NetworkTopologyStrategy
```

> When we have Cassandra running in both the nodes of datacenter, we are able to connect and create JanusGraph keyspace. However when one datacenter goes down, and if we try to open connection we observe following exception.

```
com.datastax.driver.core.exceptions.NoHostAvailableException: All host(s) tried for query failed (tried: /10.249.55.111:9042 (com.datastax.driver.core.exceptions.UnavailableException: Not enough replicas available for query at consistency QUORUM (2 required but only 1 alive)))
at com.datastax.driver.core.RequestHandler.reportNoMoreHosts(RequestHandler.java:211)
```

> We have configured `LOCAL_QUORUM`, but still why is it using `QUORUM` for establishing connection.

**nodetool status output**

```
Datacenter: dc1
===============
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
-- Address Load Tokens Owns (effective) Host ID Rack
DN 10.249.55.108 283.54 KiB 256 100.0% 619242db-f0bd-4492-aeb6-2bb0ebfe4733 rack1
Datacenter: dc2
===============
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
-- Address Load Tokens Owns (effective) Host ID Rack
UN 10.249.55.111 294.75 KiB 256 100.0% 6ebe897e-94e3-44e5-99dc-055beb633e74 rack1
```

Start with the supplied janusgraph-cassandra.properties settings and reproduce the connection attempt while dc1 is unavailable. Trace which operation requests consistency QUORUM rather than LOCAL_QUORUM, focusing on transaction-log initialization. Done means the connection succeeds with the configured local consistency and a regression test covers the two-datacenter failure scenario.

Written by the indexing model from the issue text.