GEOMESA-151 allowing schema to not be set in datastore config for already created features

Since geoserver doesn't even have the ability right now to configure a custom schema.
Would the following be a bit clearer?

```scala
val expectedIndexSchema = getIndexSchemaString(featureName)
// if there's no schema in the table, assume we will use the computed (expected) schema
val storedSchema = readMetadataItem(featureName, SCHEMA_CF).getOrElse(expectedIndexSchema)
if (storedSchema != expectedSchema) {
  errors.add(/* ... */)
}
```