GEOMESA-491 Add ability for Geoserver plugin to scan multiple columns

Changes plugin wms layer configurations to allow for a list of columns to scan
One question: in old version, there is only one columnFamily and columnQualifier for querying coverage data in Accumulo table. Above changes intends to support multiple columnFamily and columnQualifier pairs in a query. Does it mean it gets multiple coverages (surfaces) in one query? Don't see codes changes to deal with things like merging multiple coverages.