Couchbase SyncGateway should support cluster of ONE node

`IS_IN_CLUSTER` [is ONLY set when a server is explicitly added into a cluster](https://github.com/apache/incubator-brooklyn/blob/59208f56a264d85d68ccc51bc9f2acf457dc1107/software/nosql/src/main/java/brooklyn/entity/nosql/couchbase/CouchbaseClusterImpl.java#L461-L462).

The original logic checks for 'IS_IN_CLUSTER'; this means that the sole primary node of a single-node cluster will never be eligible to be used with sync gateway (even though it actually is...!).

We extend the boolean condition to check for this condition.