Unbootstrapped authentication results in NPE

In version 0.3.1, there is a potential bug in [JanusGraphAbstractAuthenticator](https://github.com/JanusGraph/janusgraph/blob/master/janusgraph-server/src/main/java/org/janusgraph/graphdb/tinkerpop/gremlin/server/auth/JanusGraphAbstractAuthenticator.java).

There is a check for the keys `CONFIG_DEFAULT_USER` and `CONFIG_DEFAULT_PASSWORD` in the configuration dictionary passed to the `setup()` method; if either key is missing, a warning is issued [(link to source)](https://github.com/JanusGraph/janusgraph/blob/2ffcb3bb690df2772693eb60d23a01976ae2a9f8/janusgraph-server/src/main/java/org/janusgraph/graphdb/tinkerpop/gremlin/server/auth/JanusGraphAbstractAuthenticator.java#L89).

However, later in the same method, the value of `config.get(CONFIG_DEFAULT_USER)` is used; `toString()` is called on it. The `config` object is a `Map`, and therefore if `CONFIG_DEFAULT_USER` is not present in it, that config lookup will return `null`. This causes the `toString()` call to throw a NullPointerException. A similar situation also occurs directly after that with `CONFIG_DEFAULT_PASSWORD`. [(link to source)](https://github.com/JanusGraph/janusgraph/blob/master/janusgraph-server/src/main/java/org/janusgraph/graphdb/tinkerpop/gremlin/server/auth/JanusGraphAbstractAuthenticator.java#L116)

This causes a rather uninformative message to be displayed during gremlin server startup:
```
9030 [main] WARN org.apache.tinkerpop.gremlin.server.AbstractChannelizer -
9031 [main] ERROR org.apache.tinkerpop.gremlin.server.GremlinServer - Gremlin Server Error
java.lang.IllegalStateException: Could not create/configure Authenticator null
at org.apache.tinkerpop.gremlin.server.AbstractChannelizer.createAuthenticator(AbstractChannelizer.java:195)
at org.apache.tinkerpop.gremlin.server.AbstractChannelizer.init(AbstractChannelizer.java:136)
at org.apache.tinkerpop.gremlin.server.channel.WebSocketChannelizer.init(WebSocketChannelizer.java:61)
at org.apache.tinkerpop.gremlin.server.GremlinServer.start(GremlinServer.java:160)
at org.apache.tinkerpop.gremlin.server.GremlinServer.main(GremlinServer.java:344)
Caused by: java.lang.NullPointerException
at org.janusgraph.graphdb.tinkerpop.gremlin.server.auth.JanusGraphAbstractAuthenticator.setup(JanusGraphAbstractAuthenticator.java:116)
at org.janusgraph.graphdb.tinkerpop.gremlin.server.auth.JanusGraphSimpleAuthenticator.setup(JanusGraphSimpleAuthenticator.java:37)
at org.apache.tinkerpop.gremlin.server.AbstractChannelizer.createAuthenticator(AbstractChannelizer.java:191)
... 4 more
9032 [main] ERROR org.apache.tinkerpop.gremlin.server.GremlinServer - Gremlin Server was unable to start and will now begin shutdown: Could not create/configure Authenticator null
```

**Reproduction:**

To reproduce this issue, add the following configuration in the gremlin-server.yaml, and launch janusgraph:
```
authentication: {
authenticator: org.janusgraph.graphdb.tinkerpop.gremlin.server.auth.JanusGraphSimpleAuthenticator,
authenticationHandler: org.apache.tinkerpop.gremlin.server.handler.SaslAuthenticationHandler,
config: {
credentialsDb: conf/gremlin-server/janusgraph-credentials-server.properties
}
}
```
(where `conf/gremlin-server/janusgraph-credentials-server.properties` points to a valid config for credentials storage)

**Recommendations**

Should the community agree that this is an issue that should be addressed, my naive thoughts going into this is to suggest that one of the following should be done to fix this:
* The logged warning is upgraded to a higher criticality level, such as `error` or `critical`. This would indicate to the user that the program will encounter issues and cannot continue execution.
* Alternatively, the value in config should be checked for null, and if it is null, change behavior such that an NPE is not thrown, or throw a more description error such that the end user can easily imply what actions to take to fix the issue based on the output given in the log.

Start in janusgraph-server/src/main/java/org/janusgraph/graphdb/tinkerpop/gremlin/server/auth/JanusGraphAbstractAuthenticator.java, especially setup() around the CONFIG_DEFAULT_USER and CONFIG_DEFAULT_PASSWORD lookups. Reproduce with the gremlin-server.yaml configuration shown and inspect startup behavior when those keys are absent. Done means missing credentials no longer causes an uninformative NullPointerException and the resulting failure or warning clearly indicates what configuration is required.

Written by the indexing model from the issue text.